Former Bosnian Serb leader Radovan Karadzic is to begin his defence at a war crimes court in The Hague.
Mr Karadzic will defend himself against 10 charges of genocide, war crimes and crimes against humanity during the war in the 1990s.
He has denied the charges, including one related to a massacre by Bosnian Serb troops at Srebrenica in July 1995.
Police arrested Mr Karadzic in 2008 on a bus in Belgrade after he had been on the run for almost 13 years.
Mr Karadzic, 67, went on trial at the International Criminal Tribunal for the former Yugoslavia in October 2009.
The single genocide charge against him relates to the deaths of more than 7,000 Bosniak (Bosnian Muslim) men and boys in Srebrenica in 1995. It was the worst atrocity in Europe since the end of World War II.
He is also being prosecuted over the 44-month siege of Sarajevo, in which more than 12,000 civilians died.
At the hearing on Tuesday, Mr Karadzic is expected to give a lengthy personal statement and to call as a first witness Col Andrey Demurenko, a Russian, who was chief of staff of the UN peacekeeping force in Sarajevo in 1995.
He is also expected to be questioned about the shelling of a market in Sarajevo in August 1995.
In June, Mr Karadzic had one charge of genocide - related to the forcible expulsion of hundreds of thousands of non-Serbs from towns and villages in Bosnia - dismissed. But he failed in his attempt to have the other charges against him dropped.
He has claimed that he did not know what was taking place on the ground.
Former Bosnian Serb army commander Ratko Mladic is also on trial at The Hague.