This paper introduces Causal Agency Theory, an extension of the functional model of self-determination. Causal Agency Theory addresses the need for interventions and assessments pertaining to selfdetermination for all students and incorporates the significant advances in understanding of disability and in the field of positive psychology since the introduction of the functional model of self-determination. Causal Agency Theory provides a theoretical framework for developing and enhancing supports to enable youth to engage in agentic action through instruction in goal setting and attainment strategies, to influence selfdetermination, causal agency, and overall well-being across diverse social-contextual contexts.