Ruto, the most senior official yet tried by the court, and his co-accused, journalist Joshua arap Sang, 38, are appearing voluntarily to face three counts of murder, deportation and persecution after a wave of violence swept Kenya in 2007-08, following a contentious election.
At the time, Ruto was a member of the Orange Democratic Movement (ODM) which enjoyed a massive following in the Rift Valley and whose main opponent was the Party of National Unity (PNU).
The Deputy President is accused of organising and financing young men to commit the crimes against Kenyans who were perceived as supporters of PNU.
Sang is on the other hand being accused of using his position at KASS FM to mobilise the young men into prosecuting and displacing these communities.