-Anthony Atkinson's proposal for a participation income (PI) has been acclaimed by social philosophers and policy experts as a workable compromise given the problems besetting an unconditional basic income (UBI). While some see PI as the first step towards a fully unconditional scheme, others regard PI as superior to UBI on ethical grounds as well as in terms of political feasibility. Both of these views disregard the administrative complications associated with introducing a broad participation requirement into welfare entitlements. In this paper we identify three essential administrative tasks that any welfare scheme must perform -establishing criteria of entitlement, determining compliance with those criteria, and allocating benefits to qualified beneficiaries -and show that PI performs poorly in terms of all three tasks. This suggests what we call the trilemma of participation income: only at significant costs to administrators and welfare clients does the scheme retain its apparent ability to satisfy the requirements of both activation and universal approaches to welfare. Consequently, the main apparent strength of PI -its capacity to unite different factions within the basic income debate -is shown to be illusory precisely because competing factions strongly prefer different resolutions of the trilemma. This, we argue, has far-reaching implications for the political strategy of basic income advocates as well as the wider debate on universal welfare reform.