<p>A worldwide movement has emerged in recent years, bringing millions of young people together to demand action on climate change. While youths’ high level of vulnerability to climate change could make them especially credible, and therefore powerful, messengers on this topic, there has been relatively little scholarly attention on <em>youth</em> activism and the nuances of framing by youth climate activists in particular. This gap may be especially important in the United States, which represents a substantial portion of global emissions but has historically struggled to establish enduring climate policy. Can this new generation of activists – many of whom are not yet old enough to vote – uniquely impact climate policy in the United States? My dissertation uses a multi-method approach to explore this question, focusing on communication by youth activists. I begin by examining the distinct frames that U.S. youth activists use to describe the issue of climate change, and exploring how they perceive those messages will influence the policymaking process. I do this using interview data with youth activists and examining Tweets, finding that youth activists often rely on climate science frames rather than justice-related frames that arguably “fit” well with their identities and vulnerabilities as youth. Next, I consider the effect of different climate frames on three sets of actors relevant to policymaking on this issue: (i) the general (adult) public, (ii) the youth public, and (iii) policymakers. More specifically, I draw on data from an original survey experiment and interviews with local- and state-level officials. I find clear evidence that the “fit” between message framing and messenger source matters – youth can be effective messengers about climate change, but particularly when they invoke arguments about the intergenerational and environmental injustices of climate inaction. The role of source identity is a critical contribution to the political communication and climate framing literature. Although many scholars have pointed to source identity as an important factor, the relationship between message content and source identity has been underexamined in the literature regarding climate change to date. This study also contributes to the framing literature through a focus on age as an important facet of source identity and by examining the causal influence of justice-based frames. Finally, this study aims to contribute to the social movement literature by a focus on the unique impacts of communication by <em>youth</em> climate activists. Given youths’ high level of vulnerability to climate impacts, this dissertation work could have notable environmental justice implications as well.</p>