This chapter examines the task of preventing harmful behaviour motivated by a violent extremist ideology. It begins with a brief review of the history of practice in the field of risk assessment and management in order to highlight the latest learning and recommendations, which the violent extremism field would do well to follow. Risk and threat assessment and management practice is then compared and contrasted in order to highlight differences that account at least in part for the wide variety of guidance and expectations currently available. The final part of the chapter focuses on practical issues in the prevention of violent extremism through its assessment, formulation, and subsequent management. Specific key issues are addressed: (i) the application of the structured professional judgement approach in practice, from triage through to extended or live risk management; (ii) the challenge of establishing a violent extremist intent in a person who is the focus of concern, in order that the most appropriate guidance and management strategies are put against his or her harm potential; and (iii) the task of embedding and evaluating harm prevention into counter-terrorism practice. The chapter ends with concluding remarks and practice recommendations.