"Collaboration-based approaches to healthcare improvement attract much attention. They involve networks of people coming together to cooperate around a common interest, with shared goals of improving care and mutual learning. Longstanding examples of collaborative approaches have been associated with some success in improving outcomes and reducing harm. The evidence for their effectiveness and cost-effectiveness, however, remains inconsistent and contingent on the circumstances in which they are deployed and how they are used for what purpose. Several models for collaboration have been developed, varying in structure, format, and balance between internal leadership and external control. We focus on two approaches: quality improvement collaboratives and communities of practice. We explore evidence of their impact on health outcomes, and evidence about how best to organise and implement collaboration-based approaches. Using examples of more and less successful collaborations, we offer guidance on the key challenges involved in using collaboration-based approaches to improve healthcare"--