We study the contribution of environmental groups to new technology adoption and diffusion by dominant incumbents. Building on institutional and social movement theory, we develop a theoretical framework that sheds light on environmental groups as change-actors. We theorize that by approaching embedded key constituents with various strategic actions, environmental groups affect taken-for-granted beliefs, build legitimacy for renewable energy technologies, and convince skeptical constituents to support the new technology. We verify our theoretical framework with a case study of wind power development in Colorado. We find that environmentalists educate constituents on the benefits of the new technology, engage in activities leading to regulatory and legislative decisions for wind, while also providing direct assistance to the utility. As a result, utilities are both pressured and encouraged to adopt and diffuse wind power on a large-scale. This research directs attention to the role of environmental groups as change-actors and the legitimating effects of their actions. It highlights their part in creating a more favorable institutional environment for new technologies while directly influencing the incumbent's technology choice. This paper contributes to an understanding of bottom-up, actor-initiated changes in energy systems taking into account both the systemic technological infrastructure and the institutional context. (C) 2013 Elsevier Ltd. All rights reserved.