BackgroundGeneral practices have begun working collaboratively in general practitioner federations, which vary in scope, geographical reach and organisational form.ObjectivesThe aim was to assess how federating affects practice processes, workforce, innovations in practices and the interface with health and social care stakeholders.DesignThis was a structured cross-sectional comparison of four case studies, using observation of meetings, interviews and analysis of documents. We combined inductive analysis with literature on ‘meta-organisations’ and networks to provide a theoretically informed analysis.ResultsAll federations were ‘bottom-up’ voluntary membership organisations but with formal central authority structures. Practice processes were affected substantially in only one site. In this site, practices accepted the rules imposed by federation arrangements in a context of voluntary participation. Federating helped ease workforce pressures in two sites. Progress regarding innovations in practice and working with health and social care stakeholders was slower than federations anticipated. The approach of each federation central authority in terms of the extent to which it (1) sought to exercise control over member practices and (2) was engaged in ‘system proactivity’ (i.e. the degree of proactivity in working across a broader spatial and temporal context) was important in explaining variations in progress towards stated aims. We developed a typology to reflect the different approaches and found that an approach consisting of high levels of both top-down control and system proactivity was effective. One site adopted this ‘authoritative’ approach. In another site, rather than creating expectations of practices, the focus was on supporting them by attempting to solve the immediate problems they faced. This ‘indulgent’ approach was more effective than the approach used in the other two sites. These had a more distant ‘neglectful’ relationship with practices, characterised by low levels of both control over members and system proactivity. Other key factors explaining progress (or lack thereof) were competition between federations (if any), relationship with the Clinical Commissioning Group, money, history, leadership and management issues, size and geography; these interacted in a dynamic way. In the context of a tight deadline and fixed targets, federations were able to respond to the requirements to provide additional services as part of NHS Improving Access to General Practice policy in a way that would not have been possible in the absence of federations. However, this added to pressures faced by busy clinicians and managers.LimitationsThe focus was on only four sites; therefore, any federations that were more active than those federations in these four sites will have been excluded. In addition, although patients were interviewed, because most were unaware of federations, they generally had little to say on the subject.ConclusionsGeneral practices working collaboratively can produce benefits, but this takes time and effort. The approach of the federation central authority (authoritative, indulgent or neglectful) was hugely influential in affecting processes and outcomes. However, progress was generally slower than anticipated, and negligible in one case.Future workFuture work would benefit from multimethod designs, which provide in-depth, longitudinal, qualitative and quantitative methods, to shed light on processes and impacts.FundingThis project was funded by the National Institute for Health Research (NIHR) Health Services and Delivery Research programme and will be published in full inHealth Services and Delivery Research; Vol. 8, No. 11. See the NIHR Journals Library website for further project information.