There is much research to support the claim that the penile plethysmograph is a reliable and valid method of objectively measuring and assessing the erectile response in male sex offenders. The authors have reviewed some of the most recent literature dealing with this subject. The conclusion is that although the penile plethysmograph is the best objective measure of male sexual arousal and can be useful in the assessment and treatment of sex offenders, caution must be exercised because of the limitations. The limitations include using the plethysmograph as a predictive test, the lack of standards for administration and interpretation, the lack standardized norms, and the susceptibility to controlling results. Despite these limitations, the plethysmograph is often used as the single most important source of predictive data in some court rooms, boards of pardons, and for prison inmate classification systems. The plethysmograph is most effective in predictive situations when it is used in conjunction with multiple data source.