The European Centre for Development Policy Management (ECDPM) has undertaken a study of capacity development, with a focus on organisational change and performance enhancement. Both individual organisations and networks of organisations have been studied with the aim of identifying important relationships among endogenous change factors (e.g. ownership, commitment and managerial style), key internal organisation variables (e.g. structures, procedures, staffing and management systems), performance and sustainability outcomes, and external environmental factors (e.g. policy frameworks, resource availability, politics, stakeholders, governance regimes, etc.). To date, the ECDPM study team has conducted 16 case studies (see Appendix). Among the findings that have emerged from several of the cases is the presence of an organisation's legitimacy as a factor contributing to successful capacity and performance. To delve in more detail into the concept of legitimacy, and to identify the implications for capacity building, ECDPM commissioned a working paper on the topic. This exploratory paper reviews the relevant literature and examines: differing definitions, types and sources of legitimacy; the links between legitimacy and organisational capacity, performance and sustainability; and management strategies for building and maintaining legitimacy. It discusses a selected set of the ECDPM cases in terms of the legitimacy concept.