A psychologist in a criminal trial is concerned with establishing by sufficient and relevant evidence that a particular event occurred, and the behaviour involved is the subject of criminal responsibility. The psychologist must provide objective opinion on matters which could not be discerned other than by someone with the appropriate expertise, based on reliable observations and/or tests. This chapter discusses the potential traps into which psychologists might fall. These include: thinking that their expertise relates to matters which are discernible without the benefit of psychological evidence; going beyond their remit; and ignoring the factual aspects of the case. The process of giving evidence is that the witness is first questioned by the side which commissioned the report about the issues on which they assert their expertise; it is followed by further questioning by the other side in the dispute, and later by further questioning by the original questioner.