Organized afterschool activities present great potential to support individuals' positive development. Despite the general findings on the positive role of organized afterschool activities on individual development, existing studies have been limited in multiple ways. They have often focused on the quantity of participation or the amount of time individuals spend in activities and only rarely on the quality of experiences in the activities although both aspects are likely important. In addition, studies have considered activities during a snapshot of development and have not examined activities across the life span. Finally, only limited empirical research has considered the potential pathways by which activities are linked to individual functioning. Situated within the bioecological perspective, the current dissertation examines the quantity and quality of experiences in organized afterschool activities and their associations with individual development. Using data from the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development (NICHD SECCYD), this dissertation consists of three studies. The first study examined the developmental pathways linking the quantity and quality of experiences in 6th grade organized afterschool activities to academic performance at the end of high school via activity participation and academic skills at age 15. Path analyses results indicated that adolescents who participated in organized afterschool activities for more days and those who reported higher quality activity experiences in 6th grade participated in organized afterschool activities for more days and reported more positive experiences at age 15. Age 15 activity participation formed an indirect pathway linking 6th grade activities to 12th grade academic performance. In addition, organized afterschool activities in 6th grade promoted academic skills at age 15, which were carried forward to 12th grade. The second study examined the association between four types of adolescent organized activities (i.e., sport, art, volunteer/community services, and religious youth groups) and leisure time activities in adulthood at age 26. Findings suggested that longer duration and higher levels of enjoyment in adolescent activities were both associated with participation in adult leisure time activities of the same type. In addition to within-type associations, cross-type association were also observed. Study 3 examined the quality of children's relationships with three adults (afterschool program staff, classroom teachers, and mothers) in 1st grade in relation to children's academic, social-emotional, and behavioral adjustment at school one year later in 2nd grade. Regression results showed that more relational conflict with afterschool staff in 1st grade was associated with lower social self-control and more externalizing behaviors one year later. Relational closeness with afterschool staff was not related to children's school adjustment the following year. Findings from the current dissertation highlight the importance of examining both quantity and quality of experiences in organized afterschool activities, as well as delineating potential pathways linking organized activity participation to individual development. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]