(CNN)Here is a look at some U.S. citizens who have been convicted of spying against the United States.
1967-1968 - Enters the Career Trainee Program at the CIA and becomes an operations officer.
1970s - Ames specializes in Soviet/Russian intelligence services.
1986-1989 - Ames is stationed in Rome and continues to pass information to Soviet agents. He is paid approximately $1.8 million during this period.
Late 1980s - The CIA and FBI learn that a number of Russian double agents have been arrested and some executed.
May 1993 - The FBI begins investigating Ames, with both physical and electronic surveillance.
February 21, 1994 - Ames and his wife, Rosario, are arrested in Arlington, Virginia, by the FBI, accused of spying for the Soviet Union and Russia. It is estimated that Ames has received approximately $2.5 million from Russia and the Soviet Union for his years of spying.
April 28, 1994 - Ames pleads guilty and is sentenced to life in prison without the possibility of parole. In October 1994, Ames' wife receives 63 months in prison.
October 31, 1995 - CIA Director John Deutch testifies before Congress about the scope of Ames' espionage. He states that more than 100 U.S. spies were compromised and that tainted intelligence was given to presidents George H.W. Bush and Bill Clinton.
1970-1991 - David Boone serves in the U.S. Army as a signals intelligence analyst. During the late 1980s, he is assigned to the National Security Agency as a senior cryptologic traffic analyst.
October 1988 - In the midst of a divorce and financial problems, Boone goes to the Soviet embassy in Washington, DC, and offers to spy on the United States. He is paid about $20,000 a year for his work over the next three years. He continues spying after being transferred to a post in Germany.
1991 - Boone loses his security clearance and retires from the Army, remaining in Germany.
1998 - He is contacted by a retired FBI agent posing as a Russian agent. The agent meets with Boone in London and the United States and pays him $9,000 to return to spying for Russia.
October 14, 1998 - Boone is charged with passing defense documents to the Soviet Union. He pleads guilty in December 1998.
February 26, 1999 - He is sentenced to 24 years in prison.
1968-1986 - Noshir Gowadia is employed by Northrop Grumman where he works on technology relating to the B-2 Spirit Bomber, aka the "Stealth" bomber.
July 2003-June 2005 - Travels to China six times to "provide defense services in the form of design, test support and test data analysis of technologies for the purpose of assisting the PRC with a cruise missile system by developing a stealthy exhaust nozzle." He is paid over $100,000 during this period.
October 2005 - Arrested and charged with passing national defense information to China. Superseding indictments are issued in 2006 and 2007.
August 9, 2010 - Gowadia is found guilty.
January 24, 2011 - He is sentenced to 32 years in prison.
January 12, 1976 - Robert Hanssen joins the FBI.
1979 - Begins spying for the Soviet Union.
1980 - Begins working for the counterintelligence unit, focusing on the Soviet Union.
1981 - Transfers to FBI headquarters, initially tracking white-collar crime and monitoring foreign officials assigned to the U.S. He is later assigned to the Soviet Analytical Unit.
1981 - Hanssen's wife catches him with classified documents and convinces him to stop spying.
October 4, 1985 - Resumes spying.
1991 - Breaks off relations with the KGB.
1999 - Resumes spying, this time for the Russian Foreign Intelligence Service.
2000 - The FBI identifies Hanssen from a fingerprint and from a tape recording supplied by a disgruntled Russian intelligence operative. The FBI also obtains the complete original KGB dossier on Hanssen.
December 2000 - The FBI begins surveillance of Hanssen.
February 18, 2001 - Hanssen is arrested in a Virginia park after making a drop of classified documents. Agents find a bag nearby containing $50,000 that they believe is Hanssen's payment for the documents.
July 6, 2001 - Pleads guilty to 15 counts of espionage and conspiracy in exchange for the government not seeking the death penalty.
May 10, 2002 - He is sentenced to life in prison without the possibility of parole.
1984 - Ana Montes is recruited to spy for Cuba. She is never paid for her spying.
1985-2001 - She is employed by the Defense Intelligence Agency as an analyst. She is promoted several times, eventually becoming the DIA's top Cuba analyst.
Fall 2000 - The FBI and DIA begin investigating Montes.
March 20, 2002 - Pleads guilty to espionage and is sentenced to 25 years in prison.
1978 - Myers travels to Cuba and is recruited by Cuban intelligence.
1979 - Myers and his girlfriend [later his wife], Gwendolyn, begin spying for Cuba. It is believed they receive little to no payment for their services.
1985 - He is hired by the State Dept. as a senior analyst.
October 31, 2007 - Myers retires from the State Dept.
November 20, 2009 - He pleads guilty to wire fraud and conspiracy to commit espionage. Gwendolyn Myers pleads guilty to conspiracy to gather and transmit national defense information.
July 16, 2010 - Myers is sentenced to life in prison. His wife is sentenced to 81 months.
1980 - Harold Nicholson joins the CIA after serving in the United States Army.
1982-1989 - Nicholson works for the CIA in the Philippines, Thailand and Japan.
1992-1994 - Deputy Chief of Station/Operations Officer in Kuala Lumpur, Malaysia.
June 1994-November 1996 - Provides Russian Intelligence with sensitive information.
November 16, 1996 - Arrested at Dulles International Airport carrying classified CIA information.
November 27, 1996 - Nicholson pleads not guilty.
June 5, 1997 - He is convicted of espionage and sentenced to 23 years in prison.
2008 - Nicholson's son, Nathaniel, is arrested on charges he met with Russian agents to collect money owed to his father.
January 18, 2011 - Harold Nicholson is sentenced to an additional eight years in prison on charges of conspiracy to act as an agent of a foreign government and conspiracy to commit money laundering. Nathaniel Nicholson is sentenced to five years probation.
1965-1979 - Ronald Pelton works for the National Security Agency, with top-level security clearance.
1979 - Pelton leaves the NSA due to financial problems.
January 1980 - After declaring bankruptcy in 1979, Pelton begins spying for the Soviet Union. He discloses classified information on the United States' ability to intercept Soviet communications.
November 25, 1985 - After a KGB defector reveals his name, Pelton is arrested and charged with espionage.
June 5, 1986 - He is convicted of spying.
December 17, 1986 - Pelton is sentenced to three consecutive life sentences plus 10 years.
1983-1996 - Earl Edwin Pitts works at the FBI.
1987-1992 - Pitts passes information on FBI operations to the Soviet Union and Russia.
1995 - A Russian diplomat at the U.N. names Pitts as a former spy. FBI agents posing as Russian intelligence officers contact Pitts in an attempt to lure him back to spying. Pitts delivers documents in exchange for $65,000.
February 28, 1997 - Pleads guilty. At the time, he is only the second agent in the FBI's history to be found guilty of espionage.
June 23, 1997 - He is sentenced to 27 years in prison.
1979 - Pollard is hired to work at the Navy Field Operational Intelligence Office. He had been rejected previously from employment at the CIA due to drug use. His specialty is North America and the Caribbean.
June 1984 - He begins spying for Israel, passing on information on Arab countries. He earns $1,500-$2,500 a month.
November 21, 1985 - Pollard is arrested outside the Israeli Embassy after his request for asylum is denied.
June 4, 1986 - Pleads guilty to conspiracy to commit espionage.
March 4, 1987 - U.S. District Judge Aubrey Robinson, Jr., rejects a plea agreement reached by federal prosecutors and Pollard. Instead, he sentences Pollard to life in prison. Pollard is the only person in U.S. history to receive a life sentence for spying on behalf of a U.S. ally. Presidents Bill Clinton, George W. Bush and Barack Obama have rejected pleas for clemency.
1995 - Israel grants Pollard citizenship.
May 11, 1998 - Israel admits for the first time that Pollard was working as its agent.
July 28, 2015 - Pollard's attorney announces that Pollard has been granted parole and will be released in November.
November 20, 2015 - Pollard is released on parole. His lawyers request the court to vacate the conditions of his parole, which include wearing an ankle monitor and monitoring his and his employer's computers.
1969-1994 - George Trofimoff, a naturalized American citizen of Russian parentage, works as a civilian for the U.S. Army at the Joint Interrogation Center in Nuremberg, Germany. He also attains the rank of colonel in the Army reserve.
1994 - Trofimoff and a priest in the Russian Orthodox church, Igor Susemihl, are arrested in Germany on spying charges. The charges are later dropped.
1994 - Retires and moves to South Florida.
June 14, 2000 - Trofimoff is arrested. U.S. Attorney Donna Bucella describes him as "the highest-ranking U.S. military officer ever charged with espionage. He is accused of passing classified information on Soviet and Warsaw Pact military capabilities from 1969-1994. Allegedly, he received payment of over $250,000 during that time.
June 27, 2001 - He is convicted of spying for the Soviet Union and Russia. He is later sentenced to life in prison.