In principle, the outcome of a criminal trial is decided by the evidence presented to the court. In reality, courtrooms play host to a complex web of people, roles and relationships, influencing one another both intentionally and unwittingly. The evidence is only one factor that determines whether a defendant is found guilty or not guilty. This chapter examines some of the psychological processes that influence a trial, starting with a consideration of different types of trial procedure and how the strength of a case may be affected by the strategies used by legal professionals. There is then a discussion of some of the legal and extralegal factors that influence jury decisions including the composition of the jury, group processes of deliberation and the impact of pretrial publicity and juror bias on the outcome of trials.