Evaluating how a client is doing in therapy is a problem faced by all therapists. This is an especially important issue when the client is a rapist or child molester, since not knowing that his control is poor or that he has failed to respond to treatment may jeopardize the safety of others. Therapists have traditionally relied on the client's selfreport to evaluate how he is doing. This traditional method of assessment in some cases is quite efficient and provides the therapist with extensive details that assist in outlining further treatment plans for the offender. In working with sexual deviates, however, it is quite apparent that in many cases this self-report is totally unreliable. As a consequence, the therapist must rely on other methods to assess his client's progress.For a number of years, penile transducer measures have been used to assess the sexual arousal of various sexual deviates. This is accomplished by the use of a small penile transducer which encircles the penis and records erection responses during the presentation of various sexual stimuli. In recent years, this method has been applied to the assessment of child molesters and sexual aggressives. Abel, Blanchard, Becker, and Djenderedjian summarize their recent work on the use of penile transducers with child molesters and rapists. The correlation of such measures with the sexual aggressive's history of sexual aggressive behavior, the frequency of that behavior, the likelihood of excessive force being used during the commission of a sexual assault, the likelihood of the client being a sadomasochist, and age of the aggressive's preferred victim is discussed. Preliminary data indicate that these physiologic measures have tremendous potential for assisting the therapist in assessing not only the treatment needs of the sexual aggressive, but also his response (or failure to respond) to treatment. valuating a patient's treatment needs and assessing his/her response to treatment has been an ever-present problem for psychiatry and psychology. These same issues develop during the assessment of patients who have raped or molested children.