International criminal procedure (ICP) has fluctuated uncertainly between common- and civil-law procedural principles. Consensus on the principles underlying ICP is needed to ensure consistent standards of justice. The article begins by comparing criminal procedure in common- and civil-law systems, and describes the theories underlying the trial and judicial role in these systems. It then compares ICP to civil- and common-law criminal procedure. This comparison establishes the scope of judicial powers that can be exercised by international criminal judges. These powers differ from those exercised by both common- and civil-law judges. The article concludes by arguing that ICP is based on a new theory of the trial: the theory of ‘tempered adversariality’.