Collaborative governance arrangements involving a diversity of groups (e.g., governments, civil society, industry) are increasingly being used to make decisions or give advice to decision-makers on water issues. Legitimacy is a critical factor for the effectiveness, efficiency, stability, and popular approval of collaborative efforts. However, as a concept, legitimacy remains contested with various meanings, theoretical backgrounds, and source norms. Clarity is particularly needed around the changing sources of legitimacy as collaborative efforts mature. Drawing on casestudy research of five collaborations in British Columbia, Canada, we present a framework of legitimacy sources as collaborations evolve. Legitimacy during the establishment of a collaboration depends principally on community readiness to collaborate, a sense of need, and the perceived potential for goal achievement. As a collaboration continues to grow, its legitimacy forms mainly from normative processesthe perceived quality of factors such as accountability, transparency, consensusbuilding, and representation of relevant discourses. Once a collaboration reaches maturity and faces questions of its future existence, legitimacy is largely resultbased-tangible and contextually meaningful outcomes must be easily identifiable and promoted. Increased understanding of the dynamics of legitimacy can help collaborative efforts strategically plan and work toward their goals as they evolve.