Prevention is an important facet of criminalization, but one that must be subject to principled constraint if it is not to result in overextension of the criminal law. This article surveys the scope and forms of prevention within criminal law, analyzes its justifications and limits, and considers whether and when prevention might better be pursued through civil or regulatory procedures. In so doing, it seeks to clarify the place of prevention within criminal law and to develop restraining principles for criminalization whose purpose is predominantly preventive.