A family therapist may become involved in the legal aspects of an incest case by treating an offender and a family as part of a probationary decree, by being a consultant in the preparation of a legal case, by testifying in a case in which the therapist has provided offender or family treatment, or by assessing a child witness' credibility. Therapists may also assist in preparing incest victims and other family members for court testimony. This paper defines the following legal terms and concepts: material witness, hearsay, expert witness, subpoena, guardian ad litem, duces tecum, trier of fact, deposition, the legal definition of incest, adjudicatory hearing, dispositional hearing, preliminary hearing, and trial. Suggestions to family therapists for testimony in incest cases cover recordkeeping and information acquisition that routinely prepare therapists for involvement in legal processes, pretrial preparation in consultation with involved attorneys, court presentation of a diagnostic assessment, qualifications of an expert witness, and the importance of using empirical evidence to support professional opinion. 24 references.