(CNN) -- Same-sex marriage is legal in the District of Columbia and six states: Massachusetts, Connecticut, Vermont, Iowa, New Hampshire and New York. Civil unions are legal in New Jersey, Illinois, Delaware, Rhode Island and Hawaii.
Many states have constitutional language defining marriage as heterosexual and statutes banning same-sex marriage.
Argentina, Brazil, Canada, and Western European countries also have laws allowing same-sex marriage and domestic partnerships.
• September 21, 1996: President 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 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 20, 2004: Sandoval County, New Mexico, issues 26 same-sex marriage licenses, but they are nullified by the state attorney general that day.
• February 12 to March 11, 2004: Nearly 4,000 same-sex couples get marriage licenses in San Francisco.
• February 24, 2004: President 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. Benton County follows on March 24.
• June 7, 2004: West 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.
• May 17, 2004: The first legal same-sex marriages in U.S. history are performed in Cambridge, Massachusetts.
• July 14, 2004: The U.S. Senate blocks a proposed constitutional amendment to ban same-sex marriage (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 gay marriage.
• October 5, 2004: A Louisiana judge throws out an amendment to the state Constitution banning gay 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 that the state's law defining marriage is unconstitutional.
• January 18, 2005: The Louisiana state Supreme Court reinstates the constitutional amendment banning gay 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 U.S. to act without a court order to sanction gay marriages.
• September 14, 2005: The Massachusetts Legislature rejects a proposed amendment to its constitution to ban same-sex marriages.
• September 29, 2005: California Gov. Arnold Schwarzenegger vetoes the same-sex marriage bill.
• October 1, 2005: A Connecticut law goes into effect that allows civil unions, and an amendment is included that defines marriage as the union of a man and a woman.
• October 7, 2005: The Massachusetts Supreme Judicial Court hears a case challenging a 1913 law prohibiting gay non-residents from marrying in Massachusetts if same-sex marriage is prohibited in their home state.
• November 8, 2005: Texas becomes the 19th state to adopt a constitutional amendment banning same-sex marriage.
• January 20, 2006: A Maryland judge rules that 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 gay marriage is legal in their home states.
• May 16, 2006: Georgia's constitutional ban on same-sex marriage is struck down in Fulton County Superior Court; a measure approved by voters in 2004 violated a rule that limits ballot questions to a single subject. (Appeals are pending.)
• May 31, 2006: Attorneys for five New York City couples plan to argue that the New York State Court of Appeals should allow the city to let same-sex couples marry because of the state constitution's guarantee of "equality, liberty and privacy for all New Yorkers."
• 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 February 19, 2007.
• January 1, 2008: The civil unions law signed by New Hampshire Gov. John Lynch in April 2007 goes into effect.
• May 15, 2008: The California Supreme Court rules that the state's ban on same-sex marriages is unconstitutional (PDF). The decision goes into effect at 5:01 p.m. June 16.
• 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 approve similar amendments to their state constitutions.
• March 23, 2009: The Vermont state Senate votes 26-4 to legalize same-sex marriages. It goes before the House of Representatives on March 27. Gov. Jim Douglas says he will veto a same-sex marriage bill if it comes across his desk.
• April 3, 2009: The Iowa Supreme Court strikes down a state law banning same-sex marriage.
• April 7, 2009: Vermont becomes the fourth state to legalize same-sex marriages after both the state Senate and House of Representatives overturn a veto by Douglas. The Senate vote is 23-5, and the House vote is 100-49.
• April 27, 2009: Same-sex marriage becomes legal in Iowa as the court ruling comes into effect. Dozens of couples across the state applied for marriage licenses, and at least three marriages were performed.
• May 6, 2009: Same-sex marriage becomes legal in Maine as Gov. John Baldacci signs a bill less than an hour after the state Legislature approves it.
• May 6, 2009: New Hampshire lawmakers pass a same-sex marriage bill. The House passes the bill in a 178-167 vote, and the Senate approves the legislation in a 13-11 vote.
• 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 remain valid.
• May 31, 2009: Nevada legalizes domestic partnerships. The state Assembly votes 28-14 to override Gov. Jim Gibbons' veto.
• June 17, 2009: President Obama signs a memorandum granting some benefits to same-sex partners of federal employees.
• July 7, 2009: A 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 the District of Columbia votes 11-2 to legalize gay marriage.
• January 1, 2010: Same-sex marriage becomes legal in New Hampshire.
• March 3, 2010: Same-sex marriage becomes legal in Washington.
• 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 Northern District Of California decides that Perry v. Schwarzenegger (Proposition 8) is unconstitutional.
• January 31, 2011: Illinois Gov. Pat Quinn signs the Illinois Religious Freedom Protection and Civil Union Act. It will go into effect June 1, 2011.
• February 1, 2011: The Iowa House of Representatives passes Joint Resolution 6, an amendment proposal banning same-sex marriage, by a vote of 62-37.
• February 16, 2011: Hawaii's Senate passes a measure to legalize same-sex unions. Gov. Neil Abercrombie says 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.
• May 12, 2011: Delaware Gov. Jack Markell signs legislation allowing civil unions between same-sex couples. The bill will take effect January 1, 2012.
• June 14, 2011: Gov. Andrew Cuomo of New York proposes the Marriage Equality Act, a bill to legalizes 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.
• June 24, 2011: The New York Senate votes to legalize same-sex marriage. Cuomo signs the bill just before midnight, and the bill goes into effect July 24, 2011.
• July 2, 2011: Gov. Lincoln Chafee signs legislation making Rhode Island the fifth state to allow civil unions between same-sex couples.
• September 30, 2011: The U.S. Department of Defense issues guidelines allowing military chaplains to perform same-sex ceremonies.
• January 30, 2012: Legislation to legalize same-sex marriage in the state of Washington passes a House committee vote and heads to the Senate. Gov. Chris 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, 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: Gregoire signs a law making same-sex marriage is signed into law in Washington state. The law will go into effect in June unless opponents halt its implementation by putting it on the November 2012 ballot.
• 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 Gov. 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 would 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 Gov. Martin O'Malley has promised to sign it into law. The law will go into effect in the summer if it survives a likely court challenge.
• 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 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.
Civil unions and domestic partnerships
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.
• Several states have granted limited marriage benefits to gays but call them domestic partnerships.
• Many cities and counties across the nation offer domestic partner registries that are open to gay couples or unmarried heterosexual couples.
Legal recognition of same-sex marriage by state (sources: Lambda Defense Fund and National Conference of State Legislatures)
Disstrict of Columbia
Civil unions or domestic partnerships
Delaware (civil unions)
Hawaii (civil unions)
Illinois (civil unions)
New Jersey (civil unions)
Rhode Island (civil unions)
California (domestic partnerships)
Oregon (domestic partnerships)
Nevada (domestic partnerships)
Limited domestic partnerships, reciprocal beneficiaries, designated beneficiaries
Colorado (designated beneficiaries)
Hawaii (reciprocal beneficiaries)
Maine (limited domestic partnerships)
Wisconsin (limited domestic partnerships)
Domestic partnership health care benefits for state employees
District of Columbia
States with constitutional amendments banning same-sex marriage
States with constitutional language defining marriage
North Dakota, 2004
South Carolina, 2006
South Dakota, 2006
Legal recognition of same-sex marriage outside the U.S. (source: Freedom to Marry Campaign)
Mexico City, 2009
South Africa, 2006