(CNN)Here's some background information about same-sex marriage in the United States and worldwide.
In the following 37 states and the District of Columbia: Alabama, Alaska, Arizona, California, Colorado, Connecticut, Delaware, Florida, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Maine, Maryland, Massachusetts, Minnesota, Montana, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin and Wyoming.
Georgia, Kentucky, Michigan, Mississippi, North Dakota, Ohio, and Tennessee.
UNDER COURT REVIEW
(Courts in the following states have declared same-sex marriage bans unconstitutional, but the rulings have been stayed) Arkansas, Louisiana, Missouri, Nebraska, South Dakota and Texas.
19 other countries (and parts of Mexico) also have laws allowing same-sex marriage. Most of these are in Europe and South America.
Civil unions grant couples most of the rights of state civil marriages, but provide none of the federal benefits of marriage, such as Social Security benefits.
These rights include spousal support, medical decision-making privileges, access to a partner's insurance, and hospital visitation rights.
Civil unions are legal in Hawaii, Illinois and Colorado.
Timeline (U.S. only):
September 21, 1996 - President Bill Clinton signs the Defense of Marriage Act banning federal recognition of same-sex marriage and defining marriage as "a legal union between one man and one woman as husband and wife."
December 3, 1996 - A Hawaiian state court ruling makes Hawaii the first state to recognize that gay and lesbian couples are entitled to the same privileges as heterosexual married couples. The ruling is stayed and appealed the next day.
December 20, 1999 - The Vermont Supreme Court rules that gay and lesbian couples should be given the same rights as heterosexual couples.
November 18, 2003 - The Massachusetts Supreme Court rules that a ban on same-sex marriage is unconstitutional.
February 12-March 11, 2004 - Nearly 4,000 same-sex couples get marriage licenses in San Francisco.
February 20, 2004 - Sandoval County, New Mexico, issues 26 same-sex marriage licenses, but they are nullified by the state attorney general the same day.
February 24, 2004 - President George W. Bush announces support for a federal constitutional amendment banning same-sex marriage.
February 27, 2004 - New Paltz, New York, Mayor Jason West performs same-sex marriages for about a dozen couples.
March 3, 2004 - In Portland, Oregon, the Multnomah County Clerk's office issues marriage licenses for same-sex couples. Neighboring Benton County follows, on March 24.
June 7, 2004 - Jason West, the mayor of New Paltz, New York, is issued a permanent injunction by the Ulster County Supreme Court against marrying same-sex couples.
March 11, 2004 - The California Supreme Court orders San Francisco to stop issuing marriage licenses to same-sex couples.
July 14, 2004 - The U.S. Senate blocks a proposed constitutional amendment to ban same-sex marriage from moving forward in Congress. (48 votes out of 60 needed)
August 12, 2004 - The California Supreme Court orders San Francisco officials, including Mayor Gavin Newsom, not to license additional same-sex marriages, pending resolution of the constitutional challenges to state marriage statutes
September 30, 2004 - The U.S. House of Representatives votes against amending the Constitution to prohibit same-sex marriage.
October 5, 2004 - A Louisiana judge throws out an amendment to the state constitution banning same-sex marriage, because the ban also includes civil unions.
November 2, 2004 - Eleven states pass constitutional amendments defining marriage as being between a man and a woman only: Arkansas, Georgia, Kentucky, Michigan, Mississippi, Montana, North Dakota, Ohio, Oklahoma, Oregon and Utah.
March 14, 2005 - A Superior Court judge rules that California's law that limits marriage to a union between a man and a woman is unconstitutional.
April 14, 2005 - Oregon's Supreme Court nullifies the same-sex marriage licenses issued there in 2004.
May 12, 2005 - A federal judge strikes down Nebraska's ban on protection and recognition of same-sex couples.
August 5, 2004 - A Washington judge rules the state's law defining marriage is unconstitutional.
January 18, 2005 - The Louisiana State Supreme Court reinstates the constitutional amendment banning same-sex marriage that was thrown out in October 2004.
September 6, 2005 - The California Legislature passes a bill to legalize same-sex marriage. The legislature is the first in the United States to act without a court order to sanction same-sex marriages.
September 14, 2005 - The Massachusetts Legislature rejects a proposed amendment to its state constitution to ban same-sex marriages.
October 7, 2005 - The Massachusetts Supreme Judicial Court hears a case challenging a 1913 law prohibiting nonresidents from marrying in Massachusetts if the marriage is prohibited in their home state. The law has been used in recent years to prevent same-sex couples from getting married.
November 8, 2005 - Texas becomes the 19th state to adopt a constitutional amendment banning same-sex marriage.
January 20, 2006 - A Maryland judge rules the state's law defining marriage is unconstitutional.
March 30, 2006 - The highest court in Massachusetts rules that same-sex couples who live in other states cannot get married in Massachusetts unless same-sex marriage is legal in their home states.
June 6, 2006 - Alabama voters pass a constitutional amendment to ban same-sex marriage.
July 6, 2006 - The New York Court of Appeals rules that a state law banning same-sex marriage is legal, and the Georgia Supreme Court upholds the state's constitutional amendment banning same-sex marriage.
November 7, 2006 - Constitutional amendments to ban same-sex marriage are on the ballot in eight states. Seven states: Colorado, Idaho, South Carolina, South Dakota, Tennessee, Virginia, and Wisconsin, pass theirs, while Arizona voters reject the ban.
December 21, 2006 - Civil unions are legalized in New Jersey. The law goes into effect on February 19, 2007.
October 10, 2008 - The Connecticut Supreme Court in Hartford rules that the state must allow gay and lesbian couples to marry.
November 4, 2008 - Voters in California approve Proposition 8, which will amend the state's constitution to ban same-sex marriage. Voters in Arizona and Florida also approve similar amendments to their state constitutions.
November 12, 2008 - Same-sex marriage becomes legal in Connecticut.
March 23, 2009 - The Vermont state Senate votes 26 to 4 to legalize same-sex marriage. It goes before the state's House of Representatives on March 27. Vermont Governor Jim Douglas says he will veto a same-sex marriage bill if it comes across his desk.
April 7, 2009 - Vermont legalizes same sex marriages after both the state Senate and House of Representatives overturn a veto by Governor Jim Douglas. The Senate vote is 23-5, while the House vote is 100-49.
April 27, 2009 - Same-sex marriage becomes legal in Iowa as the court ruling comes into effect.
May 6, 2009 - New Hampshire lawmakers pass a same-sex marriage bill.
May 26, 2009 - The California Supreme Court upholds the passage of Proposition 8, banning same-sex marriage. However, 18,000 such marriages performed before Proposition 8 will remain valid.
July 7, 2009 - A new District of Columbia law recognizing same-sex marriages performed elsewhere takes effect.
November 3, 2009 - Voters in Maine repeal the state's law allowing same-sex marriage.
December 15, 2009 - The city council of Washington, D.C. votes to legalize same-sex marriage, 11-2.
July 9, 2010 - Judge Joseph Tauro of Massachusetts rules that the 1996 Defense of Marriage Act is unconstitutional because it interferes with a state's right to define marriage.
July 15, 2010 - A federal appeals court upholds the D.C. law, rejecting an anti-same-sex marriage referendum.
August 4, 2010 - Chief U.S. District Judge Vaughn Walker from the United States District Court/Northern District Of California decides that Perry v. Schwarzenegger (Prop 8) is unconstitutional.
January 31, 2011 - Illinois Governor Pat Quinn signs the Illinois Religious Freedom Protection and Civil Union Act. It goes into effect June 1, 2011.
February 16, 2011 - Hawaii's Senate passes a measure to legalize same-sex civil unions. Gov. Neil Abercrombie says that he will sign the bill, which will take effect January 1, 2012.
February 23, 2011 - The Obama Administration instructs the Justice Department to stop defending the constitutionality of the Defense of Marriage Act in court.
April 25, 2011 - King & Spalding, the private law firm hired by House Speaker John Boehner, backs out of defending the Defense of Marriage Act.
June 14, 2011 - Governor Andrew Cuomo of New York proposes the Marriage Equality Act, a bill to legalize marriage for same-sex couples.
June 14, 2011 - U.S. District Court for the Northern District of California upholds a lower court ruling invalidating California's Proposition 8's ban on same-sex marriage as unconstitutional. The ruling had been challenged when the lower court's judge was found to be gay.
July 24, 2011 - Same-sex marriage becomes legal in New York.
September 30, 2011 - The U.S. Department of Defense issues new guidelines allowing military chaplains to perform same-sex ceremonies.
January 1, 2012 - Civil unions become legal in Hawaii.
January 30, 2012 - Legislation to legalize same-sex marriage in the state of Washington passes a House committee vote and heads to the Senate. Governor Christine Gregoire is in favor of the bill.
February 1, 2012 - The Washington state Senate passes a bill to legalize same-sex marriage, by a vote of 28-21. On February 8, 2012, the House approves the measure by a vote of 55-43.
February 7, 2012 - A three-judge panel with the 9th U.S. Circuit Court of Appeals in San Francisco rules that Proposition 8, the voter-approved same-sex marriage ban, violates the constitution.
February 13, 2012 - The New Jersey state Senate passes a bill to legalize same-sex marriage by a vote of 24-16.
February 17, 2012 - New Jersey Governor Chris Christie vetoes a bill legalizing same-sex marriage. New Jersey lawmakers have until the legislative session ends in January 2014 to override Christie's veto and will need a two-thirds majority in both houses to succeed.
February 23, 2012 - The Maryland Senate passes a bill to legalize same-sex marriage. The bill passed the Maryland House vote earlier in the month, and Governor O'Malley has promised to sign it into law. The law goes into effect in January 2013.
May 8, 2012 - North Carolina voters pass a constitutional amendment banning same-sex marriage, putting a ban that already existed in state law into the state's charter.
May 9, 2012 - Excerpts from an interview with ABC air in which President Barack Obama endorses same-sex marriage, the first such statement by a sitting president. He feels that the legal decision should be up to the states to determine.
May 30, 2012 - A lawsuit is filed in Illinois by nine same-sex couples challenging the constitutionality of a state law that denies same-sex couples the right to marry.
May 31, 2012 - The 1st U.S. Circuit Court of Appeals in Boston rules that the Defense of Marriage Act, (DOMA), discriminates against gay couples.
June 5, 2012 - The 9th Circuit U.S. Court of Appeals in San Francisco denies a request to review an earlier court decision stating that California's Proposition 8 violates the Constitution. A stay on same-sex marriages in California remains in place until the issue is exhausted in the courts.
October 18, 2012 - The 2nd U.S. Circuit Court of Appeals rules that the Defense of Marriage Act, (DOMA), violates the Constitution's equal protection clause, deciding in favor of widow Edith Windsor, an 83-year-old lesbian who sued the federal government for charging her more than $363,000 in estate taxes after being denied the benefit of spousal deductions.
November 6, 2012 - Voters in Maryland, Washington and Maine pass referendums legalizing same-sex marriage. This is the first time same-sex marriage has been approved by a popular vote in the United States. Voters in Minnesota reject a ban on the issue.
December 5, 2012 - Washington Governor Christine Gregoire signs Referendum 74, the Marriage Equality Act, into law.
December 7, 2012 - The U.S. Supreme Court announces it will hear two constitutional challenges to state and federal laws dealing with the recognition of gay and lesbian couples to legally wed. Oral arguments in the appeal are held in March 2013, with a ruling expected by late June.
December 29, 2012 - Maine's law to legalize same-sex marriage goes into effect at midnight.
January 1, 2013 - Maryland begins issuing marriage licenses to and performing ceremonies for same-sex couples as the new law goes into effect.
January 25, 2013 - The Rhode Island House of Representatives passes a bill legalizing same-sex marriage. It now goes before the state Senate for consideration.
February 14, 2013 - The Illinois Senate votes to legalize same-sex marriage. The bill now moves to the House for a vote.
March 12, 2013 - Colorado lawmakers pass legislation to legalize civil unions for same-sex couples. Governor John Hickenlooper signs it into law on March 21.
June 26, 2013 - The Supreme Court rejects parts of the Defense of Marriage Act (DOMA) in a 5-4 decision, dismissing an appeal over same-sex marriage on jurisdictional grounds and ruling same-sex spouses legally married in a state may receive federal benefits. It also rules that private parties do not have "standing" to defend California's voter-approved ballot measure barring gay and lesbians couples from state-sanctioned wedlock. The ruling clears the way for same-sex marriages in California to resume.
August 1, 2013 - Laws in Rhode Island and Minnesota to legalize same sex marriage go into effect at midnight.
August 29, 2013 - The U.S. Treasury Department rules that legally married same-sex couples will be treated as married for tax purposes, even if they live in a state that does not recognize same-sex marriage.
September 12, 2013 - A Commonwealth Court judge orders the Montgomery County, Pennsylvania Register of Wills, D. Bruce Hanes, to stop issuing marriage licenses to same-sex couples. Since July, Hanes has issued licenses to 174 couples, despite a 1996 Pennsylvania law defining marriage as between a man and a woman.
September 27, 2013 - A New Jersey state judge rules that same-sex couples must be permitted to marry in New Jersey starting October 21. The ruling says that the parallel label "civil unions," which the state already allows gay couples, is illegally preventing them from getting federal benefits.
October 10, 2013 - New Jersey Superior Court Judge Mary Jacobson denies the state's appeal to halt same-sex marriages.
October 12, 2013 - The New Jersey Supreme Court announces it will hear, in January, Garden State Equality v. Dow, to decide if New Jersey is violating the equal protection of the state's constitution by denying same-sex the right to marry.
October 15, 2013 - Brenda Clark and Carol McCrory are the first same-sex couple to receive a marriage license application in North Carolina. The law in North Carolina defines marriage between a male and a female. The application is pending approval of the state's Attorney General Roy Cooper.
October 21, 2013 - Same-sex marriage becomes legal in New Jersey. Acting Attorney General John Hoffman withdraws New Jersey's appeal in the matter of Garden State Equality vs. Paula Dow.
November 13, 2013 - Governor Neil Abercrombie signs legislation making Hawaii the 15th state to legalize same-sex marriage.
November 20, 2013 - Illinois becomes the 16th state to legalize same-sex marriage when Governor Pat Quinn signs the Religious Freedom and Marriage Fairness Act into law. The law will go into effect on June 1, 2014.
November 27, 2013 - Pat Ewert and Venita Gray become the first same-sex couple to marry in Illinois. Gray's battle with cancer prompted the couple to seek help from a federal court to receive a license before the law goes into effect in June. Gray dies March 18, 2014.
December 2, 2013 - The law legalizing same-sex marriage in Hawaii takes effect.
December 19, 2013 - The New Mexico Supreme Court unanimously rules to allow same-sex marriage statewide and orders county clerks to begin issuing marriage licenses to qualified same-sex couples.
December 20, 2013 - A federal judge in Utah declares the state ban on same-sex marriage unconstitutional.
December 24, 2013 - The 10th Circuit Court of Appeals denies a request from Utah officials to temporarily stay a lower court's ruling that allows same-sex marriage there. The ruling allows same-sex marriages to continue while the appeal goes forward.
January 6, 2014 - The Supreme Court temporarily blocks same-sex marriage in Utah, sending the matter back to an appeals court.
January 8, 2014 - State officials in Utah announce that the more than 1,000 same-sex marriages performed in the last three weeks will not be recognized.
January 14, 2014 - An Oklahoma federal court rules the state ban on same-sex marriage is, "an arbitrary, irrational exclusion of just one class of Oklahoma citizens from a governmental benefit." Anticipating an appeal, U.S. Senior District Judge Terence Kern puts in place a stay pending the outcome of the Utah appeal, so same-sex couples in Oklahoma cannot immediately marry.
February 10, 2014 - Attorney General Eric Holder issues a memo stating, "the (Justice) department will consider a marriage valid for purposes of the marital privilege if an individual is or was validly married in a jurisdiction authorized to sanction marriages, regardless of whether the marriage is or would have been recognized in the state where the married individuals reside or formerly resided, or where the civil or criminal action has been brought."
February 12, 2014 - U.S. District Judge John G. Heyburn II rules that Kentucky's denial of recognition for valid same-sex marriages violates the United States Constitution's guarantee of equal protection under the law.
February 13, 2014 - U.S. District Judge Arenda L. Wright Allen strikes down Virginia's ban on same-sex marriage as unconstitutional.
February 26, 2014 - U.S. District Judge Orlando Garcia strikes down Texas' ban on same-sex marriage, ruling it has no "rational relation to a legitimate government purpose." Garcia stayed enforcement of his decision pending appeal, meaning homosexual couples in Texas for the time being cannot get married.
March 14, 2014 - A federal preliminary injunction is ordered against Tennessee's ban on recognizing same-sex marriages from other states.
March 21, 2014 - U.S. District Judge Bernard Friedman rules that the Michigan Marriage Amendment which bans same-sex marriage is unconstitutional.
March 21, 2014 - Michigan Attorney General Bill Schuette files an emergency request for Judge Friedman's order to be stayed and appealed.
April 14, 2014 - U.S. District Judge Timothy Black orders Ohio to recognize same-sex marriages from other states.
May 13, 2014 - U.S. Magistrate Judge Candy Wagahoff Dale rules that the Idaho ban on gay marriage is unconstitutional and fails to live up to the U.S. Constitution's 14th Amendment that guarantee no "state deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws." The order is set to take effect May 16, 2014. An appeal is filed.
May 15, 2014 - The U.S. Ninth Circuit Court of Appeals responds to the appeal from Idaho. It issues a temporary stay against same-sex marriage in Idaho, which was to go into effect May 16, 2014.
May 16, 2014 - Arkansas state Supreme Court issues an emergency stay as its judges consider the appeal to the state judge's ruling for same-sex marriage. This leaves the legal status of more than 400 gay and lesbian couples who got marriage licenses in limbo.
May 20, 2014 - U.S. District Judge John E. Jones strikes down Pennsylvania's ban on same-sex marriage.
June 1, 2014 - Same-sex marriage becomes legal in Illinois.
June 6, 2014 - A Wisconsin federal judge strikes down the state's same-sex marriage ban. The judge does not immediately issue a stay of her ruling or state whether it takes effect right away. However, Wisconsin Attorney General J.B. Van Hollen says the ban is still in place and that he will appeal the ruling.
June 9, 2014 - Wisconsin Attorney General J.B. Van Hollen files a petition with the 7th Circuit Court of Appeals in Chicago to halt same-sex marriages in that state.
June 13, 2014 - U.S. District Judge Barbara Crabb temporarily blocks same-sex marriages in Wisconsin, pending appeals.
June 25, 2014 - U.S. District Judge Richard Young strikes down Indiana's same-sex marriage ban.
July 9, 2014 - A state judge in Colorado strikes down Colorado's ban. However, the judge prevents gay and lesbian couples from immediately marrying by staying his decision.
July 11, 2014 - A federal appeals court rules that about 1,300 gay and lesbian marriages performed earlier this year must be recognized by Utah.
July 18, 2014 - The U.S. Supreme Court grants Utah's request for a delay in recognizing same-sex marriages performed in late 2013 and early 2014.
July 18, 2014 - The 10th Circuit Court of Appeals upholds a judge's ruling from January 2014 that the same-sex marriage ban in Oklahoma is unconstitutional. The panel stays the ruling, pending appeal from the state.
July 23, 2014 - A federal judges rules that Colorado's ban on same-sex marriage is unconstitutional. The judge stays implementation of the ruling pending appeals.
July 28, 2014 - A federal appeals court strikes down Virginia's ban on same-sex marriage. The 4th Circuit opinion also will affect marriage laws in other states within its jurisdiction, including West Virginia, North Carolina and South Carolina. Separate orders would have to be issued for affected states in the region outside Virginia. Gay and lesbian couples in Virginia cannot marry in the state for at least another three weeks, giving the time for another appeal to be filed on enforcement.
August 20, 2014 - The U.S. Supreme Court grants a request to delay enforcement of an appeals court ruling that overturned Virginia's same-sex marriage ban.
August 21, 2014 - U.S. District Judge Robert Hinkle rules Florida's same-sex marriage ban to be unconstitutional, but same-sex marriages cannot immediately be performed.
September 3, 2014 - Federal judge Martin L.C. Feldman upholds Louisiana's ban on same sex marriages, breaking a streak of 21 consecutive federal court decisions overturning the bans since June of 2013.
October 6, 2014 - The U.S. Supreme Court refuses to hear appeals from five states -- Indiana, Oklahoma, Utah, Virginia and Wisconsin -- seeking to keep their same-sex marriage bans in place.
October 6, 2014 - Same-sex marriage becomes legal in Oklahoma, Utah, Virginia and Wisconsin.
October 7, 2014 - Same-sex marriage becomes legal in Colorado and Indiana.
October 7, 2014 - The 9th Circuit U.S. Court of Appeals in California concludes bans on same-sex marriage in Nevada and Idaho violate the equal protection rights of same-sex couples to legally marry.
October 9, 2014 - Same-sex marriage becomes legal in Nevada and West Virginia.
October 10, 2014 - The Supreme Court lifts the temporary stay on same-sex marriages in Idaho.
October 10, 2014 - Same-sex marriage becomes legal in North Carolina.
October 17, 2014 - Federal judge John Sedwick has ruled that Arizona's ban on same-sex marriage is unconstitutional and refuses to stay his ruling. The same day, Attorney General Eric Holder announces that federal legal recognition of same-sex marriages extends to Indiana, Oklahoma, Utah, Virginia and Wisconsin. The states' bans on same-sex marriages were effectively struck down as a result of the Supreme Court's decision to not review lower court rulings that had declared such ban unconstitutional. The U.S. Supreme Court rejects Alaska's request to delay enforcement of the court's ruling on same-sex marriage. A federal judge rules same-sex marriage is legal in Wyoming, with the order taking effect by October 23.
November 4, 2014 - A federal judge rules that Kansas' ban on same-sex marriage is unconstitutional. He puts the ruling on hold until November 11, to give the state time to file an appeal.
November 6, 2014 - The U.S. Court of Appeals for the Sixth Circuit upholds bans on same-sex marriages in Michigan, Ohio, Kentucky and Tennessee.
November 12, 2014 - A South Carolina federal judge strikes down the state's ban on same-sex marriage, delaying the effective date until November 20, allowing time for an appeal by the state's attorney general.
November 19, 2014 - A federal judge overturns Montana's same-sex marriage ban. U.S. District Court Judge Brian Morris' order is effective immediately.
December 15, 2014 - Florida Attorney General Pam Bondi asks the U.S. Supreme Court to extend the stay on same-sex marriages that is set to expire in early January 2015. Bondi asks that the court wait until similar appellate cases out of Ohio are decided.
January 5, 2015 - The U.S. Supreme Court denies Florida's petition to extend the stay on allowing same-sex marriages. Couples are free to marry as the case continues though the 11th Circuit Court of Appeals.
January 12, 2015 - A federal judge rules South Dakota's ban on same-sex marriage unconstitutional but stays the ruling.
January 23, 2015 - A federal court judge rules in favor of the freedom to marry in Alabama for same-sex couples but stays the ruling.
January 27, 2015 - Federal judge Callie Granade rules to strike down the same-sex marriage ban in a second case involving an unmarried gay couple in Alabama but stays her ruling for 14 days.
February 9, 2015 - Some Alabama probate judges, including in Montgomery County, begin issuing marriage licenses to same-sex couples. Others follow the instructions of Justice Moore.
February 12, 2015 - U.S. District Court Judge Callie Granade instructs Probate Judge Don Davis, of Mobile County, Alabama, to issue same-sex marriage licenses.
March 2, 2015 - U.S. District Court Judge Joseph Bataillon strikes down Nebraska's same-sex marriage ban, effective March 9. The state immediately appeals the ruling, but Bataillon denies a stay.
March 3, 2015 - The Alabama Supreme Court orders probate judges to discontinue issuing marriage licenses for same-sex couples. The judges have five business days to respond to the order.
March 5, 2015 - The 8th Circuit Court of Appeals issues a stay on Judge Batallion's ruling. The ban on same-sex marriage will remain in effect through the state's appeals process.
April 28, 2015 - The U.S. Supreme Court hears arguments in the case, Obergefell v. Hodges. The Courts ruling will decide whether States can constitutionally ban same-sex marriage.