The aim of this thesis was to explore the implementation of the mental skills training programme My Strengths Training for Life(TM) (MST4Life(TM)) delivered to young people experiencing homelessness. After reviewing the literature in Chapter 1 and providing an overview of the underpinning research philosophy and methods in Chapter 2, Chapter 3 investigated the extent to which MST4Life(TM) was delivered in the intended delivery style using observations and facilitator self-reflection forms. Informed by and building on findings from Chapter 3, Chapter 4 presents a systematic review that examined process evaluations of positive youth development (PYD) programmes for disadvantaged (or at-risk) youth. The systematic review examined the quality of the studies, methods used and barriers and enablers that these studies experienced in implementing PYD programmes to this population. Chapter 5 explored the extent to which MST4Life(TM) was delivered in the intended style from the perspective of programme participants through thematically analysing diary room entries. This chapter also explored the challenges and barriers to young people's engagement in MST4Life(TM). Chapter 6 examined the feasibility of a study protocol designed to investigate the extent to which the training was delivered as intended to frontline service staff implementing MST4Life(TM). In this study, pre- and post-training data from training recipients were also collected to examine outcomes of the training and to enhance understanding how the training was implemented. Overall, this thesis makes a novel contribution to the discipline of sport and exercise psychology by highlighting the importance of conducting process evaluations to investigate the implementation of programmes or interventions. This thesis also makes vital contributions for frontline services supporting young people at-risk of or experiencing homelessness, demonstrating key programme components to promote young people's engagement as well as providing evidence that it is possible to train frontline service staff to deliver psychologically informed programmes with fidelity to delivery style.