Today, there is an extensive body of work on deception and its detection. This chapter primarily focuses on theoretical ideas about emotional aspects of deception. A popular notion is that liars fail to suppress emotions associated with deception, such as guilt, fear, and anxiety (or other hidden emotions). This so-called leakage hypothesis has been (and is) highly influential in media, popular culture, and law enforcement. The chapter offers a critical discussion of the leakage hypothesis, including its rationale and empirical support. We find the empirical support for the leakage hypothesis severely lacking, and we are critical of its theoretical conceptualization. We conclude that although the leakage hypothesis lacks scientific support, it is an interesting phenomenon in itself. That is, it may be worth studying why the notion of emotional leakage is so influential in both scientific research and popular culture.