Countries that allow gay marriage*:
1. The Netherlands
2. Belgium
3. Spain
4. Canada
5. South Africa
6. Norway
7. Sweden
8. Portugal
9. Iceland
10. Argentina
11. Denmark
12. Brazil
13. France
14. Uruguay
15. New Zealand
16. United Kingdom
17. Luxembourg
18. Finland
19. Ireland
20. Greenland
21. United States
22. Colombia
23. Germany
24. Malta
25. Taiwan
26. Australia
27. Austria
28. Slovenia
In December 2000, the Netherlands became the first country in the world to legalize same-sex marriage. The law officially went into effect on April 1, 2001.
In February 2003, King Albert II approved Belgium's same-sex marriage bill, which had previously been passed by the Senate and Chamber of Representatives. Same-sex marriages officially began on June 1, 2003.
Canada's Civil Marriage Act took effect in July 20, 2005, making it the fourth country in the world and the first outside of Europe to legalize same-sex marriage.
Same-sex marriages in Spain, a predominantly Catholic country, officially began on July 3, 2005, despite fierce opposition from Catholic leaders, including Vatican officials.
South Africa's first legal same-sex weddings took place on Dec. 1, 2006. More than 10 years later, it is still the only African nation where same-sex marriage is legal.
Norwegian Parliament passed same-sex marriage legislation in June 2008, and on Jan. 1, 2009, Norway became the first Scandinavian country to permit gay marriages. Prior to marriage, Norway had allowed same-sex registered partnerships since 1993.
In April 2009, shortly after neighboring Norway legalized same-sex marriage, a broad majority of the Swedish Parliament voted in support of a gay marriage bill. The legislation took effect in May 2009, and in October of that same year, the Church of Sweden's board voted to allow priests to wed same-sex couples.
Icelandic Parliament voted unanimously to legalize same-sex marriage on June 11, 2010, and the law took effect later that month. The year prior, the country was the first in the world to elect an openly gay head of state: Prime Minister Johanna Sigurdardottir.
Portugal's same-sex marriage law took effect on June 5, 2010. Gay couples, however, did not have the same adoption rights as heterosexuals until 2016.
Argentina legalized same-sex marriage in July 2010, becoming the first Latin American country to do so. The country's president at the time, Cristina Fernández de Kirchner, was a strong supporter of the measure.
Denmark legalized same-sex marriage in June 2012. However, the European country was the first in the world to recognize same-sex civil partnerships back in 1989.
Uruguay permitted same-sex marriages starting in August 2013, making it the second country in Latin America, after Argentina, to do so. Uruguay had previously legalized same-sex civil unions in 2008, and gay couples were given adoption rights in 2009.
New Zealand's Parliament took the final vote to approve same-sex marriage on April 17, 2013, and the country's first same-sex weddings were permitted starting August 19 of that same year.
French President Francois Hollande signed a same-sex marriage bill into law on May 18, 2013, fulfilling a campaign promise. The law, which was officialliy enacted on May 29 of that year, also permitted gay and lesbian couples the right to adopt children.
Same-sex marriage was legalized nationwide in Brazil in May 2013, thanks to a ruling by the Justice's National Council of Brazil. The first civil unions in the country, however, took place nearly a decade prior in 2004.
Legislation permitting same-sex marriage in England and Wales passed the U.K Parliament in July 2013, and the law came into effect in March 2014. Scottland wasn't far behind -- it passed same-sex marriage legilsation in February 2014, and couples began to wed in December 2014.
Luxembourg approved a same-sex marriage bill in June 2014 by a vote of 56 to 4. The first same-sex marriages in the country were performed in January 2015, and Luxembourg Prime Minister Xavier Bettel married his same-sex partner on May 15, 2015.
The Finnish Parliament approved a bill legalizing same-sex marriage in November 2014, and the country's president signed it in February 2015. However, the law did not officially take effect until early 2017, making Finland the final Nordic nation to permit gay marriage.
In May 2015, Ireland became the first country in the world to legalize same-sex marriage through a popular referendum, with more than 60 percent of voters saying "yes" to gay marriage in the Catholic-majority country.
Greenland, an autonomous territory of Denmark, passed a bill legalizing same-sex marriage in May 2015 — nearly three years after Denmark did so.
On June 26, 2015, the U.S. Supreme Court — in the landmark Obergefell v. Hodges decision — ruled that the Constitution guarantees a right to same-sex marriage, effectively legalizing it in every state in the country. Prior to this ruling, 13 U.S. States had continued to ban it.
Colombia's Constitutional Court legalized same-sex marriage in April 2016 by a 6-3 vote.
Germany's parliament backed the legalization of same-sex marriage in June 2017, bringing the country into line with many of its European neighbors. German Chancellor Angela Merkel said she voted against the bill.
Malta's parliament voted almost unanimously to legalize same-sex marriage in July 2017. The predominantly Catholic island nation had only legalized divorce in 2011.
Taiwan's Constitutional Court ruled in favor of gay marriage in May 2017, paving the way for it to become the first place in Asia to legalize same-sex unions. The parliament of Taiwan, which the U.S. recognizes as part of China, has two years to amend existing laws or pass new ones.
On Dec. 5, 2017, Austria's Supreme Court paved the way for same-sex marriage, ruling a distinction between marriage and registered partnership can no longer be upheld. Same-sex ouples are expected to be able to marry in Austria starting in 2019.
On Dec. 7, 2017, the Australian Parliament voted to allow same-sex marriage across the nation. The vote took place just weeks after the majority of Australians said "yes" to gay marriage in a non-compulsory postal vote. Same-sex couples across the country will be permitted to tie the knot starting Jan. 9, 2018.
Slovenia’s same-sex marriage law took effect in February 2017, but gay couples are not permitted to adopt in the European Union country. For this reason, some LGBTQ advocacy groups do not consider Slovenia to have officially legalized same-sex marriage.
*Including the territory of Greenland (Denmark) and the state of Taiwan
Sources: NBC News, Pew Research Center, Reuters, ILGA
NBC News / Interactive: Jiachuan Wu, Roque Ruiz