The thesis of this article is that the presence of medicine and behavioral science in sex offender commitment schemes is central to their claims of validity. It is the systemic, rather than casual or interstitial, presence of science that provides the central pillar in the claim for legitimacy. The article describes the operation of sex offender commitment laws, introduces their legitimacy claims through the use of science, and discusses how the actual use patterns of science undercuts those claims. In addition, the article reviews the literature on the use of science in law; details ways sex offender commitment schemes use and fail to use science; recommends uses of science in sex offender commitments; and raises a set of cautions about even this use of science in law. The purpose of science is to expose the structure of the choices open to courts, to demonstrate the necessity of, and provide the tools for, a principled translation between the languages of law, policy, and morals, on one hand, and the language of science, on the other. Notes