Prevailing conceptions of criminal responsibility and punishment and the resultant participation of mental health professionals in the criminal process are under sweeping attack. Many critics of the insanity defense assert that the law ought to be indifferent to most claims of abnormal psychological functioning. They argue that punishment should be based primarily on the offending conduct, and that the ethic of individualization should be abandoned. This challenge to subjectivism has been most pronounced in connection with the predictive inquiries characteristic of sentencing and parole systems. It is argued herein that the insanity defense as it now exists will probably be maintained, and that the trend toward subjectivism and allied clinical inquiry will increase. Decisions regarding appropriateness of incarceration will continue to be based partly on predictive criteria. Given this premise, it is necessary to examine the proper role of medical testimony in criminal proceedings. The wholesale exclusion of expert witness testimony from reconstructive inquiries would unduly prejudice the interests of defendants and enhance the natural advantage enjoyed by the prosecution, contrary to fairness principles. Expert testimony is essential to the establishment of a workable diagnosis, to explaining mental elements of offenses, and to describing behavior in terms of insanity and diminished responsibility. Evidentiary rules which permit 'informed speculation' by qualified clinical experts enable defendants to assist triers of fact to assess the plausibility and significance of subjective defenses. The primary focus of reform efforts should be on improving the quality of clinical participation in the criminal process through affording the evaluator maximum access to relevant, reliable information about the subject and facilitating objectivity in the interpretation of the information. Footnotes with references are included in the article.