Rehabilitative interventions for adult gang members are notably lacking within the criminal justice system. Existing research suggests that more general programmes underpinned by Risk-Need-Responsivity Principles may be insufficient to address the complexities associated with gang membership, meaning specialist intervention is warranted. Where many scholars have focused on gangs as a youth phenomenon, understanding of the psychological processes that comprise the complex trajectories of those who extend their membership into adulthood is limited. With this in mind, the thesis adopts a qualitative approach to explore the individual trajectories of gang members, illuminating the psychological processes associated with change in this population to appreciate and comprehend their experience. Firstly, a systematic review of the qualitative literature identifies the mechanisms of change which underpin existing gang intervention programmes. Using meta-ethnography to synthesise data from 11 studies, ‘Identity change’ was identified as a mediating variable of gang interventions facilitated by the following mechanisms; ‘Possible selves’, ‘Self-Construction vs Self-Discovery’ and ‘Immersion into roles.’ The practical implications of using these mechanisms to inform a sequential framework for the delivery of gang intervention programmes are discussed. Secondly, the narratives of five ex-gang members were analysed using interpretative phenomenological analysis to explore the meaning attributed to their experience as they transitioned in and out of the gang. Super-ordinate themes of ‘Positioning self in a social world’, ‘Solutions to identity, meaning and belonging, and ‘Catalysts for change’ shed light on the similarities and divergence across their experience and are discussed in respect of existing literature. Implications for policy, practice and future research are also discussed.