Broadly, there are two avenues for challenging the testimony of psychiatrists and psychologists. One is to challenge both the level and the quality of expertise within these disciplines; another is to question the conduct of the evaluation, including the experts' errors of omission or commission and those conclusions that are unwarranted by the supporting data or by the state of knowledge. Within psychiatry and psychology, there is a substantial body of literature, consisting of research findings and statements by authorities, which details deficiencies and problems in the knowledge base. Some themes in this literature are that experienced clinicians are no more accurate in their evaluations than inexperienced clinicians, and methods used by psychiatrists and psychologists to evaluate people are fraught with problems that render the resulting data and conclusions doubtful. Regarding methodological flaws, a common omission is the failure to obtain information from sources other than the litigants. Also, an evaluation for custody purposes that does not include an evaluation of the children is inadequate. It is also inadequate if the clinician fails to observe interactions between the parents and the children. This article offers cross-examination questions that pertain to theory and principles, reliability and validity, and the clinical examination.