If we understand development as an emergent property of a complex system, then effective development assistance needs to adapt and evolve in-context. This thesis explores how learning and adaptation practices might help aid organizations apply complexity thinking to improve their effectiveness. Based on a new framework of organizational practices, this study uses a mixed methods approach to assess the extent to which 12 small and medium international aid organizations in Haiti learn and adapt. The study supports the assumption that learning and adaptation contribute to effectiveness, and finds that organizations vary significantly in their learning and adaptation practices. It finds that development organizations employ more learning practices than humanitarian assistance organizations, and that organizations are generally better at collecting information and adopting learning attitudes, than they are at establishing the structures and processes they need to be truly adaptive. The research also finds that the barriers that make learning and adaptation more difficult for organizations are largely structural and related to aid system dynamics, while organizations benefit from enablers that are largely attributed to individual agency. This thesis argues for the important role that aid organizations can, and must play in making aid more effective – at the project, organization, and aid system levels. However, the aid system itself does not encourage learning. International aid organizations will therefore need to actively engage in learning if they are to play an effective role in development, and be a meaningful part of the system-level aid effectiveness dialogue.