Child sexual abuse is often extremely difficult to prove. Molestation occurs in secret, and the child is usually the only eyewitness. While many children are capable witnesses, some give ineffective testimony and some cannot even take the stand. In order to comprehend the dynamics of sexual abuse cases and to appreciate the utility of expert testimony, judges and attorneys must be familiar with medical and psychological literature on the subject. Rules of evidence governing expert testimony vary slightly from State to State, although broad uses and limits of such testimony are similar. The authors look at the admissibility of expert testimony; expert testimony based on novel scientific principles; medical evidence of child sexual abuse; and behavioral science testimony describing behavior commonly observed in sexually abused children on whether a child has been sexually abused, on the credibility of child witnesses, and on the profile of persons who abuse children. It is concluded that courts should proceed cautiously when considering the admissibility of expert testimony on child sexual abuse. It is vitally important that professionals offering such testimony be highly qualified. Courts should insist on a thorough showing of expertise before permitting individuals to testify as experts. Further, courts should require the proponent of expert testimony to lay a complete foundation so that the court understands precisely how the evidence is relevant. 633 footnotes