Background: This study explored the way families support self-determination in young adults with intellectual and developmental disabilities (IDD) during life transitions. Method: Qualitative case studies were conducted with two Canadian families who participated in semi-structured interviews and ethnographic observations every quarter for one year. Analyses were informed by family systems theory and self-determination theory. Findings: Families considered the needs and preferences of the young adults with IDD, suggesting individualized approaches for balancing independence and protection. Families set short-term and long-term goals for increased independence, scaffolded the learning of new skills and collaborated on important choices. Collaboration occurred to the extent that all family members perceived agency in planning and implementing transitions. Conclusions: Families supported the young adults with IDD in their psychological needs for competence, relatedness and autonomy, which allowed them to experience self-determination. Findings have implications for supporting self-determination and transition planning in the family system.