Social media has been a part of election campaigns for more than a decade. In this special issue, we combine longitudinal and cross-national studies of social media in election campaigns, expanding the time span as well as number of countries compared to former comparative studies. The four papers present examples of longitudinal studies, covering multiple election cycles from four different countries: Australia, the United States of America, Denmark, and Italy. By including the countries mentioned, we focus on countries considered to be “first movers” when it comes to the digitization and internetization of the political life. As such, they are “most similar cases.” However, they also have different political systems: the United States and Australia are characterized by a Westminster system dominated by a few large parties and a tradition of strong confrontation between government and opposition, whereas Denmark and Italy are multi-party systems with a tradition of collaboration and coalition governments. Technologically, the four countries might be similar, but politically and in terms of media systems, they differ; the United States is characterized by a commercialized American media system with little role for public service broadcasters, Denmark has very strong public service media, and Australia has elements of both these systems. Finally, Italy represents a Southern European media system with traces of clientelism as well as public service media. Thus, studies of the four countries form a diverse yet solid set of cases for exploring the growing (and changing) role of social media in national elections.