Judges in Taiwan are due to decide whether it will become the first place in Asia to recognise same-sex marriage.
The top court will rule on a constitutional challenge to current laws, which say that marriage is between a man and a woman.
If they are deemed unconstitutional, the parliament will be forced to amend the laws or pass new ones.
Wednesday』s decision comes as the LGBT community faces increasing persecution in the region.
Self-ruled Taiwan, which China claims sovereignty over, is known for its liberal values and holds the biggest annual gay pride event in the region.
Momentum for marriage equality has been building since last year, when President Tsai Ing-wen, who is openly supportive of the move, came to power.But the debate has prompted a backlash, with mass protests by conservatives in recent months.
One of the two parties that has petitioned the court to make this ruling is long-time equal rights activist Chi Chia-wei.
"I feel 100% confident about a positive outcome," he told AFP news agency.
"I am optimistic but I wouldn』t be overly excited. This should have happened long ago," he added. Mr Chi made his first appeal for recognition of gay marriage in 1986.
The second party is the Taipei city government, which is itself being sued after having to reject marriage applications from same-sex couples under existing laws.
If the 14-judge panel rules in favour of the legal challenge, the parliament, known as the Legislative Yuan, will begin the process of amending the laws. They could either legalise same-sex marriage or introduce new separate civil partnership legislation.
A bill to legalise same-sex marriages is already making its way through parliament, but that process has slowed because of opposition from traditionalists, who do not want Taiwan to become the first place in Asia to allow such weddings.