The Substance Abuse and Mental Health Services Administrations Center for Substance Abuse Prevention (CSAP) is responsible for identifying and disseminating scientifically defensible knowledge about proven prevention models and principles to the substance abuse prevention field. This booklet is intended to assist prevention practitioners and others involved in the design, implementation, and evaluation of substance abuse prevention programs. This document provides the following: describes the evolution of the conceptual model that CSAP uses to provide a unifying framework for substance abuse prevention; discusses the principles that provide the foundation for understanding and defining research findings as scientifically defensible; and illustrates how CSAP applies specific principles and criteria to research studies, program evaluations, and scholarly efforts to identify scientifically defensible findings, prevention principles, and prevention models. The guide also provides specifics about the CSAP methodology for selecting model programs and identifying scientifically defensible principles. It is one in a series of three products that together assist prevention practitioners, evaluators, state and local program administrators, policymakers, and funders in designing and assessing scientifically defensible programs. (Contains 25 references.) (GCP)