Political parties in Australia and the United Kingdom have begun to experiment with primary elections, similar to those in the United States, to select candidates for public office. Primaries, or ‘community pre-selections’, mark a distinctive shift away from previous methods of candidate selection that have typically privileged party members and elites. They extend participation in a process of selection that has traditionally been closed to members of the public. In this article, I analyse the implementation of primaries in these democracies through the broader lens of party organisational reform. I identify the key characteristics of these trials (such as the selectorate, method and turnout), the actors involved in the process and their motivations for advocating change. I adopt a three-level approach, showing that reform is driven by factors at the level of individual parties, party systems and by elite perceptions of changes at the level of the political system.