Relatively enduring institutional parameters that might influence the policy process have been incorporated into the ACF under the umbrella of “coalition opportunity structures” (COS). The concept can help policy scholars better understand the role of institutional factors in the policy process, including coalition formation, behavior, and ultimately policy change. However, the notion of COS needs theoretical refinement as well as empirical measurement. This chapter theorizes about how COS enable or constrict the ability of actors to influence policy change by reinforcing, challenging, and, in some cases, overturning the policy status quo. In addition, we address the dynamic nature of COS, which can lead to changes in the institutional arrangements within which coalitions compete to shape policy. We explore our theory by examining the historic impact of COS on nuclear facility siting in the US.