This literature review was commissioned by the National Foundation for Educational Research (NFER) to draw together current and recent studies of integrated working, in order to build an overview of the theories and models of such working. The review is important for current work on evaluating the early impact of integrated children's services and informing the direction of further work in this area. The review aimed to: (1) collate and analyse findings on models and theories of multi-agency working, integration and collaboration from relevant sources identified in recent and current NFER literature reviews in these areas; (2) collect any further relevant sources that involve model and theory building in integrated services, and analyse these sources to audit examples of theories and models of integrated working; and (3) present an overview or meta-analysis of models and theories in the area of integrated working. The report therefore represents a synthesis and meta-analysis of the most relevant evidence of models and theories of integrated working, primarily drawn from children's services research, but also including other transferable research on integrated working that focuses on models and theoretical analyses. Criteria for inclusion in the review included: (1) a focus on empirically based research (rather than descriptions of practice); (2) a focus on models and theories of integrated working/services (for example, multi-agency, collaborative, partnership, interagency and so on) within children's services; (3) inclusion of other research on integrated working that focuses on models and theoretical analyses, not captured in the above reviews, for example, concerning adult services; (4) literature published from 2000 onwards; and (5) literature that refers to the UK context (although some international literature has been included if particularly pertinent). Thirty-five sources met the criteria. The review findings are presented under thematic headings which reflect the main dimensions of the models and theories of integrated working. These dimensions comprise: (1) the extent of integration: the "stage" or depth of the collaborative activity in integrated services; (2) the integration of structures: layers of an organisation's functioning, for example, governance and strategic levels, and frontline operational service delivery levels; (3) the integration of processes: the ordering of work activities across time and place, at different organisational levels; and (4) the reach of integration: the extent to which partnerships in integrated services reach out to include diverse agencies. Each section of the report discusses different models of integration and identifies the challenges, enablers and impacts across each of the main dimensions. Overarching issues that arise from the integration of services are also discussed. Appended are: (1) Definition of terms relating to multi-agency activity; and (2) Search strategy. (Contains 2 figures.)