This textbook analyses the origins and effects of insider risk, using multiple real-life case histories to illustrate the principles, and explains how to protect organisations against the risk. Some of the most problematic risks confronting businesses and organisations of all types stem from the actions of insiders – individuals who betray trust by behaving in potentially harmful ways. Insiders cause material damage to their employers and society, and psychological harm to the colleagues and friends they betray. Even so, many organisations do not have a systematic understanding of the nature and origins of insider risk, and relatively few have a coherent and effective system of protective security measures to defend themselves against that risk. This book describes the environmental and psychological factors that predispose some individuals to become harmful insiders, and the most common pathways by which this happens. It considers how aspects of insider risk have been altered by shifts in society, including our increasing reliance on technology and changes in working patterns. The second half of the book sets out a practical systems-based approach to personnel security – the system of defensive measures used to protect against insider risk. It draws on the best available knowledge from industry and academic research, behavioural science, and practitioner experience to explain how to make personnel security effective at managing the risk while enabling the conduct of business. This book will be essential reading for students of risk management, security, resilience, cyber security, behavioural science, HR, leadership, and business studies, and of great interest to security practitioners.