The importance of risk assessment cannot be overstated, and in this chapter we explore why it is considered the cornerstone of effective correctional programs. How risk and need factors are determined, what are considered the major factors associated with the risk of reoffending, and how assessment figures into correctional interventions are all reviewed. Over the years, risk assessment has progressed from “gut feelings” to fourth-generation tools that can be used to form case plans designed to target criminogenic needs. While risk assessment is not without its problems, good assessment practices allow correctional professionals to identify “who” should receive the most intensive programming and supervision as well as “what” should be targeted in order to reduce the likelihood of recidivism.