Despite reforms of rape statutes, proof of lack of consent continues to be critical to successful prosecution. Because of jurors' attitudes, circumstantial evidence of lack of consent is very important, and two types of scientific evidence have proved valuable -- evidence of the complainant's physical condition and evidence of the victim's psychological condition. The American Psychiatric Association now includes posttraumatic stress disorder in its Diagnostic and Statistical Manual of Mental Disorders, and research confirms that RTS can be categorized as a posttraumatic stress disorder. Clinicians agree that reaction to rape consists of two phases: an acute phase charcterized by complete disruption of the victim's life followed by a long-term phase of reorganization. Legal recognition, however, is currently a subject of debate. A review of the standards of admissibility for expert scientific testimony shows that RTS evidence of lack of consent satisfies even the general acceptance and reliability rules of the strict Frye standard. The paper discusses circumstances that would be both favorable and unfavorable to the admissibility of expert testimony on RTS, as well as the positive and negative effects of admitting such testimony. RTS testimony is most effective when a highly qualified expert has examined a complainant who does not have an extensive history of mental illness and has found that she exhibits sufficient symptoms of RTS to allow a diagnosis of posttraumatic stress disorder. The paper includes 364 footnotes.