This dissertation research examines how innovation is sustained through processes of legitimation and collective action. While much is known about innovation, less is known about how it is sustained. To address this issue, I use a longitudinal, qualitative approach to focus on what actors do to sustain innovation in a technically and organizationally complex project. I draw on institutional and social movement scholarship to develop theory on strategies and mechanisms that sustain innovation. I investigate this issue by examining a sustained innovation project at the National Aeronautics and Space Administration (NASA). I focus on one of the projects in NASA's portfolio, the Space Infrared Telescope Facility (SIRTF), which was under development from 1971–2003. To develop a deep understanding of the project and its place in the overall organization, I prepared a book-length, peer-reviewed history of SIRTF, in which I trace the project's level of legitimacy with various audiences—institutional, organizational and project stakeholders. The history is a scholarly account constructed from primary data including interviews with key participants, and secondary data including meeting minutes and diaries of the project team. Based on these data I conducted two sets of analyses. In the first study, I examine how material objects are used to gain legitimacy. I find five unique properties of material strategies, thereby extending theory on legitimation strategies in sustained innovation. In the second study, I examine how the project team mobilizes resources and shape the institutional arrangements for innovation. I draw on social movement theory to develop five mechanisms by which the project team sustains innovation. Taken together, these findings contribute to theory on the strategies and mechanisms by which actors sustain innovation in long-term, complex projects.