A syndrome is a set of symptoms that indicates a particular condition. Expert testimony on a psychological syndrome can be used to explain past behavior, forecast future behavior, measure current capacity, or discover the causes of a current condition. Federal Rule of Evidence 702, governing the admissibility of expert testimony, allows judicial discretion and hinges upon whether the testimony will aid the factfinder in understanding certain evidence or in resolving a factual issue. When expert testimony is based on novel scientific theories, courts frequently apply special tests of admissibility which supplement the codified rules, notably the 'Frye' test, which conditions the admissibility of a theory on its general acceptance within the relevant scientific community. Case law does not provide evidentiary principles upon which courts consistently rely to handle syndrome evidence. This article's proposal for dealing with syndrome evidence recommends renewed emphasis on the underlying reliability of a particular theory, including replacement of the 'Frye' test with 11 factors listed by McCormick; liberal interpretations of the 'helpfulness' requirement for expert testimony; and a focus on the weight of evidence rather than its admissibility when determining if the jury should hear the testimony. 193 footnotes.