Relapse is a common part of the recovery process for individuals with substance use disorders and is often defined as any return to substance use after a period of abstinence. Relapse occurrence interrupts the recovery process and places increased pressure on substance-related care and support services. Research focusing on relapse has suggested potential factors which may influence the return to substance use however the overall research on relapse, specifically within recovery services, is limited. The overarching aim of this thesis, therefore, was to advance the understanding of relapse, with a focus on factors which influence the occurrence of relapse for those in substance-related recovery support. This thesis was carried out in collaboration with Pobl Ltd who provide substance-related care and support services in South Wales, UK. Chapter two presents study one which is comprised of a comprehensive systematic review. This systematic review examines the literature surrounding factors predicting drug and alcohol relapse. Chapter three involves interviews with service-providers to explore their views and professional experiences of substance-related relapse. All service-providers included in this study worked directly with service-users who were engaged in recovery care and support services. Interviews with service-users were conducted in chapter four to gain an insight into their personal experiences of substance-related relapse and to highlight the specific factors which they believe played a role in their relapse occurrence. This third study comprised of service-users currently involved in recovery care and support services. The final study presented in chapter five utilised a mixed methods methodology. Service-user responses to a 15-item recovery monitoring tool were analysed and visually represented into radar charts. An interview was then conducted with the aftercare worker who examined each individual service-users’ radar chart to provide a deeper insight into the accuracy of the monitoring tool in identifying changes they had experienced throughout their follow-up responses. The overall findings of this thesis have advanced relapse research by identifying predictors of substance misuse relapse within UK substance-related care and support services. Study one, which examined the substance misuse relapse literature, identified cue-response, craving, impulsivity, and depressive symptoms as key relapse predictors. In study two service-providers established maladaptive coping, limited support, behavioural changes, external exposure to use, and readiness to change as predictors of relapse. Similarly in study three, service-users identified coping with negative events, limited support from recovery services, and external exposure to substances as relapse predictors as well as social separation and rationalised return to use. Study four findings provided an evaluation of Pobl’s recovery monitoring tool and found it to have limited effectiveness in assessing the risk of relapse among service-users without valuable insight from the aftercare team and their familiarity with the service-users. The practical implications of findings for Pobl recovery services and other substance misuse recovery support services are discussed along with thesis limitations focusing on inconsistencies of defining relapse and methodological weaknesses. Future research directions discussed include establishing a standardised definition of substance misuse relapse, expanding the sample size of service-providers and service-users to include a more diverse range of participants, and utilising the findings of relapse predictors from this thesis to develop a tool for identifying relapse risk among service-users. The identification of substance misuse relapse predictors has practical implications for designing targeted interventions and providing tailored support to individuals at risk of relapse and therefore improving the effectiveness of substance-related services and promoting sustained recovery.