In common with most other mass democratic parties the Conservative party has a large group of active members who sustain the party over time. A model is developed to explain variations in activism within the party, which takes account of the ‘paradox of participation’. The results, based on the first national random sample survey of Conservative party members, show that activism is motivated by three classes of factors. Activism is motivated, firstly, by a variety of selective incentives, such as ambitions for elective office. It is motivated, secondly, by a desire for the party to achieve policy goals. These are ‘collective goods’, which are subject to the problem of free-riding. However, since activists can influence policy outcomes, via their contacts with party leaders, they have high levels of personal efficacy and a direct incentive to participate, which can override the paradox of participation. Finally, activism is motivated by expressive concerns, as measured by the strength of the respondent's partisanship, a motivation for involvement which lies outside a narrowly cast rational choice model of political participation.