The exploration of reasonable medical certainty by forensic psychiatrists appears to be of relatively recent origin, spanning approximately the past quarter of a century. While undoubtedly psychiatrists and other physicians involved in the legal arena had at least thought about the significance of reasonable medical certainty prior to this time period, there appears to have been no significant discussion in the psychiatric literature prior to the views presented by three contemporary forensic psychiatrists: Seymour Pollack, Jonas Rappeport, and Bernard Diamond. The types of clinical questions that arise in forensic psychiatric evaluation, whether in the criminal or civil arena, span the gamut of psychiatric knowledge. An encyclopedic work would be needed to attempt coverage of each forensic topic. A sampling of issues relevant to reasonable medical certainty in the chapter so as to illustrate some of the challenges encountered in forensic psychiatric practice.