The Frye test for the reliability of expert testimony requires that the relevant research measure what it purports to measure, that the same results are achieved by all researchers, and that the scientific principles underlying the testimony have general acceptance in the relevant scientific community. The literature overwhelmingly supports the conclusion that most, if not all, sexual assault victims experience a predictable selection of symptoms from the array now known as rape trauma syndrome. Federal rules of evidence require that expert testimony be relevant to the matter before the court, that it not be unfairly prejudicial, that it be built on a proper foundation, and that the expert have the proper credentials. Expert testimony on rape trauma syndrome can be both relevant and helpful to the trier of fact in rape cases. When the defense of consent is proposed, such testimony can be useful circumstantial evidence which tends to disprove that the victim acquiesced in sexual intercourse. Regarding its prejudicial character, expert testimony on rape trauma syndrome has none of the inflammatory content that typifies unfair prejudice to a criminal defendant. 157 footnotes.