(CNN)Here's a look at some of the most important cases decided by the U.S. Supreme Court since 1789.
Situation: Federalist William Marbury and many others were appointed to positions by outgoing President John Adams. The appointments were not finalized before the new Secretary of State James Madison took office, and Madison chose not to honor them. Marbury and the others invoked an Act of Congress and sued to get their appointed positions.
The Court decided against Marbury 6-0.
Historical significance: Chief Justice John Marshall wrote, "An act of the legislature repugnant to the constitution is void." It was the first time the Supreme Court declared unconstitutional a law that had been passed by Congress.
Situation: Dred Scott and his wife Harriet sued for their freedom in Missouri, a slave state, after having lived with their owner, an Army surgeon, in the free Territory of Wisconsin.
The Court decided against Scott 7-2.
Historical significance: Slaves are not citizens and thereby cannot sue in federal court. The decision overturned the Missouri Compromise, where Congress had prohibited slavery in the territories. The Dred Scott decision was overturned later with the adoption of the 13th Amendment, abolishing slavery in 1865 and the 14th Amendment in 1868, granting citizenship to all born in the U.S.
Situation: While attempting to test the constitutionality of the Separate Car Law in Louisiana, Homer Plessy, a man of 1/8 African descent, sat in the train car for whites instead of the blacks-only train car and was arrested.
The Court decided against Plessy 7-1.
Historical significance: Justice Henry Billings Brown wrote, "The argument also assumes that social prejudice may be overcome by legislation and that equal rights cannot be secured except by an enforced commingling of the two races... if the civil and political rights of both races be equal, one cannot be inferior to the other civilly or politically. If one race be inferior to the other socially, the Constitution of the United States cannot put them upon the same plane." The Court gave merit to the "Jim Crow" system. Plessy was overturned by the Brown v. Board of Education decision.
This decision overturned Plessy v. Ferguson and granted equal protection under the law.
Situation: Segregation of the public school systems in the United States was addressed when cases in Kansas, South Carolina, Delaware, and Virginia were all decided together under Brown v. Broad of Education. Third-grader Linda Brown was denied admission to the white school a few blocks from her home and was forced to attend the blacks-only school a mile away.
The Court decided in favor of Brown unanimously.
Historical significance: Racial segregation violates the Equal Protection Clause of the 14th Amendment.
This decision guarantees the right to counsel.
Situation: Clarence Earl Gideon was forced to defend himself when he requested a lawyer from a Florida court and was refused. He was convicted and sentenced to five years for breaking and entering.
The Court decided in favor of Gideon unanimously.
Historical significance: Ensures the Sixth Amendment's guarantee to counsel is applicable to the states through the 14th Amendment's due process clause.
Situation: The New York Times and four African-American ministers were sued for libel by Montgomery, Alabama, police commissioner L.B. Sullivan. Sullivan claimed a full-page ad in the Times discussing the arrest of Dr. Martin Luther King, Jr., and his efforts toward voter registration and integration in Montgomery were defamatory against Sullivan. Alabama's libel law does not require Sullivan to prove harm since the ad did contain factual errors. He was awarded $500,000.
The Court decided against Sullivan unanimously.
Historical significance: The First Amendment protects free speech and publication of all statements about public officials made without actual malice.
Situation: Ernesto Miranda was convicted of rape and kidnapping after he confessed, while in police custody, without benefit of counsel or knowledge of his constitutional right to remain silent.
The court decided in favor of Miranda 5-4.
Historical significance: Upon arrest and/or questioning, all suspects are given some form of their constitutional rights - "You have the right to remain silent. Anything you say can and will be used against you in a court of law. You have the right to an attorney. If you cannot afford an attorney, one will be provided for you. Do you understand the rights I have just read to you? With these rights in mind, do you wish to speak to me?"
Situation: "Jane Roe" (Norma McCorvey), single and living in Texas, did not want to continue her third pregnancy. Under Texas law, she could not legally obtain an abortion.
Historical significance: Abortion is legal in all 50 states. Women have the right to choose between pregnancy and abortion.
Situation: President Richard Nixon's taped conversations from 1971 onward were the object of subpoenas by both the special prosecutor and those under indictment in the Watergate scandal. The president claimed immunity from subpoena under executive privilege.
The Court decided against Nixon 8-0.
Historical significance: The president is not above the law. After the Court ruled on July 24, 1974, Richard Nixon resigned on August 8.
Situation: Allan Bakke had twice applied for and was denied admission to the University of California Medical School at Davis. Bakke was white, male, and 35 years old. He claimed under California's affirmative action plan, minorities with lower grades and test scores were admitted to the medical school when he was not, therefore his denial of admission is based solely on race.
The Court decided in Bakke's favor, 5-4.
Historical significance: Affirmative action is approved by the Court, schools may use race as an admissions factor. However the Equal Protection Clause of the 14th Amendment works both ways in the case of affirmative action; race can not be the only factor in the admissions process.
2012 - Dept. of HHS v. Florida (11-398); Florida v. Dept. of HHS (11-400); Natl. Federation of Business v. Sebelius (11-393) - Healthcare Reform
Situation: The constitutionality of the sweeping health care reform law championed by President Barack Obama.
Historical significance: The ruling upholds the law's central provision - a requirement that all people have health insurance
This decision ruled that the Defense of Marriage Act, which defined the term "marriage" under federal law as a "legal union between one man and one woman" deprived same-sex couples who are legally married under state laws of their Fifth Amendment rights to equal protection under federal law.
Situation: Edith Windsor and Thea Spyer were married in Toronto, Canada in 2007. Their marriage was recognized by New York state, where they resided. Upon Spyer's death in 2009, Windsor was forced to pay $363,000 in federal taxes, because their marriage was not recognized by federal law.
The court voted 5-4 in favor of Windsor.
Historical significance: The court rules that the Defense of Marriage Act is unconstitutional.
1967 - Loving v. Virginia - Prohibition against interracial marriage was ruled unconstitutional, 9-0 for Loving.