Clinicians are commonly asked to participate in the determination of an individual's mental capacity and susceptibfity to undue influence. Vague statutory definitions and lack of operational criteria for both determinations have contributed to inconsistency in the quality and usefulness of expert input to these determinations. Questionnaire responses of 119 probate judges from the U.S. indicate that this input is quite influential despite the problems mentioned above. This paper presents and discusses the judges' views on selected procedural and conceptual aspects of expert evaluation and testimony.The legal system employs concepts called "mental capacity" (MC) and "susceptibility to undue influence" (SUI). Despite