(CNN) -- Here's a look at what you need to know about same-sex marriage in the United States and worldwide.
Facts: Same-sex marriage is legal in 16 U.S states and the District of Columbia: California, Connecticut, Delaware, Hawaii, Illinois, Iowa, Maine, Maryland, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Rhode Island, Vermont, and Washington.
Same-sex marriage is banned by constitutional amendment or state law in: Alabama, Alaska, Arizona, Arkansas, Colorado, Florida, Georgia, Idaho, Indiana, Kansas, Kentucky, Louisiana, Michigan, Mississippi, Missouri, Montana, Nebraska, Nevada, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, South Dakota, Tennessee, Texas, Utah, Virginia, West Virginia, Wisconsin and Wyoming.
New Mexico has no laws either banning or allowing same-sex marriage.
Worldwide, 16 other countries (and parts of Mexico) also have laws allowing same-sex marriage and domestic partnerships. Most of these are in Europe and South America.
Civil Unions: 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: 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 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 the same day.
February 12-March 11, 2004 - Nearly 4,000 same-sex couples get marriage licenses in San Francisco, California.
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 U.S. 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.
September 29, 2005 - California Governor Arnold Schwarzenegger vetoes the same-sex marriage bill.
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.
May 15, 2008 - The California Supreme Court rules that the state's ban on same-sex marriages is unconstitutional. The decision goes into effect on June 16th at 5:01pm.
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 - 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.
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.
June 17, 2009 - President Barack Obama signs a memorandum granting some benefits to same-sex partners of federal employees.
July 7, 2009 - A new District of Columbia law recognizing same-sex marriages performed elsewhere takes effect.
September 1, 2009 - Same-sex marriage becomes legal in Vermont.
November 3, 2009 - Voters in Maine repeal the state's law allowing same-sex marriage.
December 15, 2009 - The city council of Washington, DC votes to legalize same-sex marriage, 11-2.
January 1, 2010 - Same-sex marriage becomes legal in New Hampshire.
March 9, 2010 - Same-sex marriage becomes legal in Washington, DC.
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 DC 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.
June 24, 2011 - The New York Senate votes to legalize same-sex marriage. Governor Andrew Cuomo signs the bill just before midnight.
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 6, 2012 - Same-sex marriage becomes legal in Washington.
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.
May 2, 2013 - Rhode Island Gov. Lincoln Chafee signs a bill legalizing same-sex marriage after the state legislature approves the measure. It goes into effect in August 2013.
May 7, 2013 - Delaware legalizes same-sex marriage. It goes into effect in July.
May 14, 2013 - Minnesota Gov. Mark Dayton signs a bill giving same-sex couples the right to marry. The law goes into effect in August 2013.
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 doesn't 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.
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, passed by the Senate yesterday, making Hawaii the 15th state to legalize same-sex marriage. The law will go into effect on December 2.
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 14, 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.