Communication campaigns play a key role in shaping what people think, feel, and do about climate change, and help shape public agendas at the local, national, and international levels. As more people around the world gain regular access to the Internet, online and social media are becoming significant contexts in which they come into contact with—or fail to come into contact with—news, debates, action, and social input related to climate change. This makes it important to understand the campaigning that takes place online. Many actors make concerted efforts to engage publics on climate change and go online to do so. These include businesses; governments and international organizations; scientists and scientific institutions; organizations, groups and individuals in civil society; public intellectuals and political, religious and entertainment leaders. Not all are ultimately concerned with climate change or engaging publics as such. Nevertheless, most campaigns involve at least one of four goals: to inform, raise awareness, and shape public understanding about the science, problems, and politics of climate change; to change consumer and citizen behavior; to network and connect concerned publics; to visibly mobilize consumers or citizens to put pressure on decision-makers. Online climate change campaigns are an emerging phenomenon and field of study. The campaigns appeared on broad front around the turn of the millennium, and have since become increasingly complex. In addition to the elements that produce variance in offline campaigns, scholars examine the role of online and social media in how campaigners render the issues and pursue their campaigns, how publics respond, and what this means for the development of the broader public discourse. Core debates concern the capacity and impact of online campaigning in the areas of informing, activating and including publics, and the ambivalences inherent in leveraging technology to engage publics on climate change.