Some 20,000 people died during Croatia's war of independence
The International Court of Justice (ICJ) in The Hague is to announce its ruling on genocide cases between Croatia and Serbia.
The Croatian government has alleged that Serbia committed genocide in the town of Vukovar and elsewhere in 1991.
Serbia later filed a counter claim over the expulsion of more than 200,000 Serbs from Croatia.
About 20,000 people died during the 1991-1995 war, when Croatia it broke away from Yugoslavia.
Croatia filed its initial case with the ICJ - the top UN court - in 1999, accusing Serbs of targeting ethnic Croats during the conflict.
The town of Vukovar was devastated when it was occupied by Serbs in 1991.
Croatia also wants Serbia to pay compensation for damages "to persons and properties as well as to the Croatian economy and environment".
- Act committed with intent to destroy in whole or part a national, ethnical, racial or religious group
- Killing members of the group
- Causing serious bodily or mental harm to members of the group
- Deliberately inflicting conditions of life calculated to bring physical destruction
- Imposing measures intended to prevent births
- Forcibly transferring children of the group to another group
In 2010, Serbia responded with a countersuit, saying that some 230,000 ethnic Serbs had been forced to flee when Croatia launched an operation in 1995 to retake territory captured by Serbs.
The BBC's Anna Holligan in the Hague says that for some this legal judgement will help to shed light on what actually happened during the darkest years of the Balkans.
Although genocide is the most serious of international crimes, it is also the hardest to prove, our correspondent adds.
The ICJ is to decide whether either country or both are guilty of genocide, incitement to commit genocide or complicity in genocide.
Both sides have said they will accept the rulings.
Relations between the two countries have improved in recent years but in 2012 Serbia was outraged when Operation Storm commander Ante Gotovina, was cleared on appeal by the International Criminal Tribunal for the former Yugoslavia (ICTY).