Relational aspects of professional practice demand increasing attention in research on work and learning. However, little is known about how knowledge is enacted in practices where different people work together. Working in partnership with clients surfaces a number of epistemic demands, responses to which are poorly understood. This paper analyses two cases of nurses working with parents in support services for families with young children. The questions asked are: What epistemic practices are enacted when professionals work in partnership with clients? How do they generate distinct modes of partnership work? Findings show how professionals' and clients' knowledge is mobilised and made actionable through practices of diagnostic reasoning, recontextualising, testing and contesting knowledge claims. A distinction is presented between partnership that unfolds as strengthening the client from a professional epistemic perspective, and that which validates and augments the client's own epistemic contribution. This reveals how knowledge is made to matter and becomes a basis for action in the course of working with others, and informs a new analytical distillation highlighting key epistemic aspects of professional-client partnership.