In many settings phallometric evaluations of sexual arousal are routinely conducted with sexual offenders and these evaluation procedures also serve as research instruments. There are, however, problems with the psychometric bases of these assessments, and studies reporting their use have so many idiosyncratic features that comparisons are of dubious value. Evidence concerning the reliability and criterion validity of phallometric testing leaves a lot to be desired, although the research has suggested an important but limited value in predicting subsequent recidivism. Suggestions are made for further research and for the clinical use of phallometric assessments within more comprehensive evaluations of sexual offenders.