The existence of gangs can no longer be regarded as an urban myth in Europe (Klein, Kerner, Maxson, & Weitekamp, 2001). There is a growth in literature on the presence of gangs in metropolitan areas across the UK (e.g. Bradshaw, 2005; Everard, 2006; Shropshire & McFarquhar, 2002). To date, gang research has been primarily criminological and sociological in nature (Bennett & Holloway, 2004; Wood & Alleyne, 2010), and since criminological theories pay scant attention to the social psychological processes involved in joining a gang (Thornberry, Krohn, Lizotte, Smith, & Tobin, 2003) there is a real need to understand more about the psychology of gang involvement (Wood & Alleyne, 2010). To that end, this thesis sheds light on the psychological processes that underpin gang membership and gang- related crime. While the purpose of this thesis, is not to test theory, Thornberry and colleagues' (2003) Interactional Theory was used to improve understanding and make educated inferences about the relationships between gang involvement and its correlates. Four studies were conducted concurrently. The first study laid the foundation by illustrating the social context in which gangs manifest and sustain themselves. Study two showed how attitudes, perceptions, and cognitions interact with varying levels of gang involvement providing insight into the development of gang members. Study three demonstrated how psychological processes work hand- in-hand with social factors to reinforce the gang culture. Finally, study four addressed the behavioural outcome of gang involvement, gang-related crime, by examining its predictors and correlates. These four studies are discussed in the context of theory development, and prevention/intervention programmes and policy. In summary, the findings of this thesis expand on the current literature by uniquely examining the role of psychological processes that elaborate on why young people become involved in gangs. These findings also highlight areas for future research.