This book introduces Casual Agency Theory as a framework for defining self-determination and describing its development and essential characteristics. It synthesizes the research on promoting and enhancing self-determination and explores implications for practice in a disability context across the life course. The book also highlights future directions for theory, research, and practice in areas of positive psychology and disability.  Key areas of coverage include: Theoretical basis of the development of self-determination as well as research findings and implications for practice across the life course. Issues relating to the implementation of evidence-based practices to promote self-determination in a disability context. Potential strategies for overcoming barriers to self-determination through research-based practices. Future directions for further integrating Casual Agency Theory into positive psychology interventions and the systems of supports in the disability field. Self-Determination and Causal Agency Theory is an essential resource for researchers, professors, and graduate students as well as clinicians, therapists, and other professionals in the fields of developmental and positive psychology, educational policy and practice, special education, social work, occupational, speech, and language therapy, public health, and healthcare policy.