Humanitarian organizations are in a privileged position to observe what happens in war. No surprise then that as tribunals –intentional or otherwise- are set up to try atrocities committed during war, those tasked with the bringing the perpetrators to justice turn to humanitarian organizations to see what evidence they can offer. For many such organizations, and the people they employ who may have personally witnessed atrocities, the question of whether or not to co-operate with prosecutions is contentious. This article sets out the legal tools available to humanitarian organizations to better manage any co-operation with international tribunes. It also addresses such questions as, can humanitarian organizations be forced to testify? Can they hand over information on a confidential basis? And if they do give evidence, can this be kept out of the public domain?