This chapter investigates the use of risk assessment instruments for sex offenders. Over the past 20 years, numerous risk assessment strategies and processes have been developed to assist clinicians in evaluating an individual’s risk to reoffend. Currently, actuarial and structured professional judgment are the competing and leading approaches in the risk assessment literature. Actuarial approaches are often described as mechanical, mathematically driven algorithms. Risk factors are explicit and fixed, and there is no professional or clinical input into what risk factors are selected or how they are weighed. Structured professional judgment (SPJ) approaches, however, are a guide rather than a formula. SPJ tools not only guide an assessor to consider empirically identified risk factors but also allow clinical flexibility with respect to how specific risk factors are weighed and how risk is ultimately determined. Despite the empirical strength of actuarial risk measures, SPJ tools have been broadly adopted, given their utility in risk management planning and clinical decision-making.