Why do people lie, and how can lies be detected? There is now a substantial psychological literature relating to these fundamental questions, and this book reviews the relevant knowledge in detail, before focusing on guidelines for best practice in detecting deception. Psychological research is now available on individual differences in lying behaviour (gender differences, age differences and personality). There is also interesting research evidence of the ways in which deception is reflected both in real objective non-verbal behaviour and also in the perceived non-verbal cues which can help or mislead the observer in detecting deception. Although the book does include a major survey of the physiological aspects of deception and the polygraph as a method of detection, it also includes a thorough review of current knowledge of content analysis and validity assessment of speech and written statements. The book ends by discussing how professionals can improve lie detection by focusing on key aspects of the behaviour of the liar and by awareness and control of their own behaviour. It covers all three aspects of deception-non-verbal cues, speech and written statement analysis, and physiological responses. It focuses on the behaviour and perceptions of the observer which can hinder the process of detection. Based on the authors expert review of the research and evidence, and on his practical experience and connections with several police forces, this book is an important contribution to research and practice in lie detection. Research about verbal, nonverbal and physiological correlates of truth telling and deception are reviewed comprehensively in one text. This book will benefit those who have to decide whether people are telling the truth or lying, because it both reviews contemporary research and provides practical guidelines.