This paper reclaims the concept of solidarity for democratic theory. It does this by proposing a theory of solidarity as social involvement that is construed through the integration of three better known conceptions of solidarity that have played an influential role in the political thought of the last two centuries. The paper begins by explaining why solidarity should receive more sustained attention from political theorists with an interest in democracy, and proceeds by presenting two indispensability arguments. Section three outlines the (...) three rival conceptions of solidarity and contends that whilst individually incomplete, each provides an important insight, so that a fuller and more satisfying conception of solidarity can be developed by weaving together some features of these three conceptions. This task is undertaken in section four, which introduces the theory of solidarity as social involvement, defining solidarity in terms of acting-for and acting-with. Section five briefly discusses some of its potential implications for democratic theory, before bringing the article to a close. (shrink)