Purpose This knowledge synthesis aimed to 1) Map the extent and nature of the literature on capacity building in the field of rehabilitation for transition-age youth with disabilities (12-30 years old) and 2) Describe how capacity building is conceptualized and identify principles and key ingredients underpinning this concept. Materials and Methods A scoping review using JBI methodology was employed. A search of six databases resulted in 2169 English documents; 34 were retained. Two reviewers charted and analyzed the data, supported by the third reviewer. Inductive content analysis was used to identify principles and key ingredients. Results Seven documents provided explicit definitions of capacity or capacity building. Content analysis revealed four principles describing capacity building as: 1) individualized approach with real-world application 2) fostering a preferred future 3) youth taking ownership for change and 4) an ongoing process. Six key ingredients detail how to build capacity: 1) individualized and flexible approach in natural context 2) shared responsibility 3) use of accessible information and resources 4) cultivate strengths 5) opportunities for full participation and 6) facilitate reflection on experiences. Conclusion Clinicians and researchers can draw upon identified capacity building principles and ingredients to support meaningful real-world outcomes for transition-age youth.