This thesis charts a course through an emerging landscape of online user participation in archives, focusing upon user involvement at the point of practice known to professional archivists as archival description. Recent years have seen significant growth in participatory initiatives in the archive sector, and the application of Web 2.0 technologies — augmenting traditions of user engagement and volunteering — has been widely heralded as a new opportunity to ‘democratise’ archival practice. The study considers a spectrum of online initiatives which have sought to benefit from the skills or knowledge of diverse user groups: from mass participation ‘crowdsourcing’ transcription projects, via tagging and commenting functionalities added to traditional archive catalogues, to community engagement programmes which have attempted to build up multiple layers of narrative interpretation. The research was designed around three principal stakeholder groups, professionals, participants, and users, seeking to address three main research questions: • Does online user participation constitute an evolution or a revolution in archival practice and professionalism? • What contexts and circumstances motivate and sustain participation? • Who benefits from user participation in archival description? Two new analytical frameworks are presented as navigation aids for this exploration of participatory archives, taken from the perspective of professional archivists and of participants respectively. The discussion on users is necessarily more speculative, but concludes that realisation of the claims made for the transformative impact of online user participation is dependent upon a redefinition of archival use which is inclusive of both participation and the communication of meaning, in addition to the routine processes of information seeking. Future research directions are identified therefore which lie at the points of intersection between engagement (participation and use combined) and professional theory and practice