This article uses the theory of 'communities of practice' to explore the discontinuity of knowledge sharing across different groups co-located within a collaborative research partnership. It presents the findings of a qualitative case study conducted within one of the Collaborations for Leadership in Applied Health Research and Care (CLAHRCs)-large-scale UK-based knowledge mobilization initiatives bringing together the producers and users of health research. Focusing on the boundaries emerging between and within the research and implementation strands of the CLAHRC, the article describes how differences between communities of practice give rise to discontinuities in knowledge sharing. Its findings highlight the role of fragmented organizational design, divergent meanings and identities, and dysfunctional boundary bridges in the (re)production, legitimization, and protection of boundaries between groups. Finally, the article questions the role of research implementation as a boundary practice bridging the gap between academic research and clinical practice.