After identifying nine characteristics that define nontraditional psychological evidence, the article distinguishes major categories of nontraditional psychological evidence thus far used in the courts and describes the substance of each type of testimony, the uses to which each has been put, and its current admissibility status. The article then examines 'quasi' nontraditional psychological evidence, which would be nontraditional except that it is associated with the insanity defense. Next, the shortcomings of traditional approaches to the admissibility of nontraditional psychological evidence are noted. The author proposes a four-factor balancing test for the admissibility of nontraditional psychological evidence: necessity (how necessary the testimony is in the particular case), reliability (accuracy and consistency), understandability (jurors' ability to understand and give proper weight to the testimony), and importance (for the resolution of the case). Once the court has analyzed these four factors, it must then balance them to determine the likelihood that the evidence will assist the jury. 389 footnotes.