Psychiatrists and other mental health professionals are frequently involved as expert witnesses in court proceedings related to children and adolescents. Their testimony may be based on a therapeutic relationship, but frequently arises because of an assessment conducted specifically for the court process. This two part paper discusses some of the issues that arise when child psychiatrists are involved as expert witnesses in litigation, with specific focus on their role in child custody, sexual abuse and young offender cases. It also offers some practical advice for those who may be called as witnesses. There is controversy in the legal profession about the role of mental health professionals in the court process. While there is recognition of their expertise, there is also a concern about not wanting to have experts usurp the role of the courts. Legal professionals also question the “objectivity” of experts, and the reliability of their opinions. Frequently the opinions of psychiatrists about children and adolescents involved in litigation have inherently speculative and value based dimensions, and not “scientific”. Participation in the court process by mental health experts is nevertheless a vitally important role, providing information, analysis and recommendations about what are often very difficult societal decisions. Part two of this paper starts on page 531.