The credibility of child witnesses is discussed, along with research in identifying the symptoms and characteristics of intrafamily child sexual abuse. Federal Rules of Evidence applicable to cases dealing with expert testimony in child sexual abuse cases are identified, and the manifestations of the sexually abused child syndrome are discussed. Several cases permitting varying degrees of expert testimony on child sexual abuse syndrome are detailed. The article suggests that an expert witness should include in his testimony information for the court and the jury about the syndrome, detail general characteristics that a sexually abused child displays, and tell whether the particular victim's behavior is consistent with the syndrome. If they are to make an informed judgment, jurors must be told whether the victim's symptoms correspond with the syndrome's criteria. The expert's testimony should be subject to cross examination by the defense. 172 footnotes.