This article addresses issues surrounding the role of public participation in expert-driven biodiversity monitoring and research, reviewing a range of cross-disciplinary insights and critiques that are important for recent debate in environmental geographies. The paper identifies normative, instrumental and substantive motivations dimensions of such initiatives and examines the tensions within these. A key focus concerns the ‘win–win’ model of public participation in scientific research (PPSR); claims of multiple benefits from PPSR, such as increased knowledge of biodiversity issues and of participants’ local environments; claims that doing PPSR is a form of ‘social learning’; and suggestions that engagement in science will change attitudes and environmental behaviour. The ‘win–win’ model is found to veil important issues about the politics of knowledge. These include the framing of citizenship in ‘citizen’ science, the production of certain kinds of scientific subjects within PPSR, the framing of relationships between professional and non-professional parties, assumptions about the role of ‘data’ in the rational evidenced-based process and anxieties amongst professional scientists around relations between data quality and the breadth of participation. Whilst the affective engagement with subject and the non-human world in PPSR is rich and diverse and the expert, non-expert boundary a mutable one (particularly in natural history), there is increasing contention that the win–win model for PPSR only works if we overlook aspects of these knowledge politics.