This contribution introduces Current Sociology's special subsection 'The advent of the citizen expert: Democratising or pushing the boundaries of expertise?', which deals with the increasing involvement of 'ordinary citizens' as experts into political and social debates. From an integrated perspective that transcends policy fields and societal realms, the special subsection deals with the epistemic and democratic implications of this transformation in civic participation and knowledge validation practice. It pays special attention to the tensions that can be implied by citizen expertise's 'double promise' of tapping into novel channels of participation and idle knowledge resources at the same time. Three promising themes and research avenues are identified that the advent of the citizen expert highlights: The changes in liberal-democratic culture indicated by the emergence of this new actor category, the way societal power relations are impacted by the elevation of citizen expertise and the subsequently shifting boundaries and standards of what can count as knowledge or expertise.