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. In reality, the jurors may understand neither the evidence presented to them nor the legal instructions given by the judge and they may be unable to separate the courtroom evidence from their own biases and their other knowledge of the case. Psychologists have investigated a number of processes that may affect jury decision making. The chapter discusses 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 pre-trial publicity and juror bias on the outcome of trials. According to Bornstein and Greene, about 90% of the time the majority verdict of individual jurors before deliberation is the same as the jury's group verdict after deliberation.