Systematic review supplement (table S1) presents key SNA terms, along with their associated deﬁnitions, theories and propositions. Work on the diffusion of medical innovations by Becker9 and Coleman et al10 has conﬁrmed the impor- tance of local peer inﬂuences or social networks. While several reviews of social and organisational networks exist in the non-health sector literature,11e13 this review focuses on the health sector, in particular on health professional networks. Drawing from a literature review of complex socio-technical systems, Braithwaite et al14 argued for exploitation of natural network characteris- tics to achieve safer, better healthcare. Similarly, Parchman et al15 contend that efforts to understand the delay in adoption of evidence-based guidelines have been hindered by an overreliance on the attributes, knowledge, decision-making, and actions of individual clinicians, and an under-recognition of the network of care within which they operate. Despite considerable progress in understanding what networks are, how they are structured, how they operate, and how they develop, we still know little about their effectiveness and sustainability in the health sector or their contributions to quality of care and patient safety.16 For example, Provan and Milward17 note the scarcity of comparative network data that are tied to outcomes, citing work by Lehman et al18 and Provan and Milward,19 while Provan and Kenis20 highlight the critical role of network governance and its impact on network effec- tiveness. This review examines the empirical research on the structure of networks of health professionals, with regard to the effectiveness and sustainability of networks, especially in relation to quality of care and patient safety. METHODS The systematic review was part of a broader review of the literature on social-professional networks of health professionals from 1995 to 2009.21 This period was selected because most of the empirical work on health professional networks has been published since 1995, spurred on by advances in computing capacity and enabled through the development of SNA software. The search strategy (ﬁgure 1) obtained a subset of 26 articles bearing on network structure. The literature search was conducted between September and December 2009 using ﬁve electronic databases: MEDLINE, CINAHL, EMBASE, Web of Science (Science Citation Index, Social Science Citation Index, Arts & Humanities Citation Index) and Business Source Premier (Management & Business). Trial searches were undertaken with a number of additional electronic databases (LISA, Scopus, ABI- Inform-Global, IBSS, EconLit), however these did not yield additional papers and were not included in the 240 search. We did not include the ‘grey literature’ as it did not meet the quality criteria of being peer reviewed and published in scholarly journals. Following a preliminary review of terms in the litera- ture, and of the MeSH database deﬁnitions of terms, key search terms were selected by the researchers to identify published research literature on social networks of health professionals (see online supplement, table S2). The key search term utilised was ‘social network’ for the initial search yielding 14 607 articles. As the intent was to explore the literature relating only to social networks of health professionals, additional terms in this table were used to reﬁne the search. To narrow the review to rele- vant material, a two-stage approach was used so that the articles generated from the ﬁrst stage, the initial ‘social network’ search, were then examined in combination with each of the subsequent terms, in separate searches. Search articles were reviewed to remove duplicates and incomplete references, yielding 1560 articles. The titles and abstracts were examined independently by two reviewers using inclusion and exclusion criteria. Under inclusion criteria, research had to focus in some depth on one or more aspects of networks of practising health professionals, or health agencies, particularly with rele- vance to quality of care and sustainability; that is, mere mention of the term ‘network’ was not sufﬁcient. Other inclusion criteria included empirical research, peer reviewed, English language, scholarly journals, human and abstract and full text available. Exclusion criteria included articles on social networks of patients, clients or caregivers; health service networks (with no relevance to health professional practice relationships); non-health professionals (except for those working in the health system, health administrators or health policy makers); internet social networking; student-education processes; academic professionals or research scientists; infection control or epidemiological networks; bio-networks and neural networks; and e-health systems and software not relating to health professional practice. This yielded 66 articles, obtained in full text for independent review by the two reviewers. By drawing on published checklists,22e25 quality was assessed according to the following: whether there was a clear and system- atic description of the aim of the study, participants, sampling strategy, data collection and analysis methods, results of the study, relationship between the researchers and the participants, context and setting of the study, strengths and weaknesses, and implications of the study. (The study quality assessment criteria and ratings, the inclusion criteria and review process are set out in the online supplement: table S3 and boxes S1 and S2). Studies were excluded only after discussion among at least two reviewers, who assessed and agreed on the inclusion and quality rating of the studies. From this BMJ Qual Saf 2012;21:239e249. doi:10.1136/bmjqs-2011-000187