Changes in American healthcare over the last half century have created social and economic crises, presenting challenges for doctors and patients. The recently-implemented Patient Protection and Affordable Care Act is an incremental reform that does little to change the complex multi-payer financing characterizing American healthcare. There have been growing demands for more equitable financing arrangements, notably, a single-payer healthcare system in which medical care is financed through a single, non-profit payer and in which medical care is treated as a public good and medically-necessary care is available to everyone. Nationally-representative surveys have demonstrated widespread physician support for single-payer legislation. Yet, very little scholarship has examined physician activism and virtually no studies have examined physician activism for single-payer healthcare. It is important to examine physician activism for single-payer because their participation is considered fundamental to achieving the goals of the movement. If the movement is successful in implementing single-payer financing , more efficient use of healthcare resources will ensure that all residents have access to needed medical care without being saddled by financial burdens from their care. Oregon is one of several US states with a growing grassroots movement to enact single-payer healthcare at the state level. This study seeks to examine the determinants of collective action for physicians in the Oregon movement for single-payer healthcare by answering two research questions. First, what accounts for differences in activism among physicians who support single-payer healthcare system? And second, for those physicians who are active, what activities do they do and what shapes those choices of activities? Data includes 21 semi-structured interviews with physicians around the state of Oregon supplemented with participant observation data. The interview data was analyzed using techniques from grounded theory and thematic analysis. I find that among collective action theories, collective identity theory best accounts for whether or not a physician engages in single-payer activism. A strength of collective identity theory is that it brings to light the importance of subjective interpretations of structural conditions by movement actors. The findings suggest that differences in interpretation shape the influence of motivators for and barriers to an individual's decision to engage in activism. Physicians that become active are primed to engage in single-payer activism because of their moral value sets and frustrating work experiences. They seek out groups of like-minded physicians who then are part of the process of socially-constructing a collective identity. This collective identity is emotionally-laden, is a reaction to state policies, serves to distinguish insiders from outsiders, and facilitates activism. Activist physicians engaging in the process of collective identity come to believe that altering financing is the only way to solve healthcare system issues. The activists view the political and cultural barriers to single-payer as surmountable by their activism. In contrast, non-activists interpret structural conditions like American politics and American culture as immutable barriers that will prevent the attainment of single-payer at the national or state level. In addition, non-activists lack the collective identity activists share because their beliefs contradict key beliefs of activists. The combination of the lack of collective identity and the perception of immutable barriers results in their non-participation.