This thesis aimed to explore factors that can influence the likelihood of adolescents’ participation and dropping out of team sports. Study I was a systematic review and meta-analysis of factors associated with dropout from team sports among adolescents. The results showed that constructs related to motivation and years of sports experience had the strongest relationship with adolescents’ team sports participation and dropout. Study II aimed to investigate how combinations of multiple demographic and motivational factors were associated with the risk of dropout in adolescent soccer players using a prospective design. The results indicated that older age, experiencing less autonomy support from the coach, less intrinsic motivation, being female, and a lower socioeconomic status were factors associated with an increased dropout risk. In Study III, the aim was to explore adolescents’ experiences of participating in and dropping out of team sports. A substantial grounded theory was created through coding and constant comparison. The theory ‘Losing fun’ illustrates adolescents’ experiences of participating and dropping out of team sports, which involved navigating demands, perceiving changed demands, and re-evaluating. When adolescents’ navigation of demands makes them perceive changed demands, it initiates a process of re- evaluating their sport. This involves questioning the meaningfulness of their sport and struggling with negative emotions. Finally, when team sports are evaluated as mainly associated with negative emotions and not meaningful anymore, adolescents regard it as not fun and are likely to drop out. Based on the fun integration theory, Study IV aimed to understand what makes soccer fun for adolescents in Sweden by investigating their prioritisation of the fun factors and determinants and comparing sub-groups in the sample (i.e., based on gender, age, perceived competence, and sports program type). The results showed that Trying Hard, Positive Team Dynamics, Positive Coaching, and Learning and Improving were the highest ranked concerning what makes soccer fun, irrespective of gender, age, perceived competence and sports program type. The study findings highlight that facilitating positive social experiences that promote feelings of effort and development is crucial to promoting fun and continued participation in soccer for most adolescents.