Background: Methods are evolving to synthesise evidence about health problems and the options for addressing them; partly driven by the growing awareness of the complexity of many research questions and the need for a broad range of primary research evidence and methods to synthesize that evidence. The dominance of particular forms of review question and method, and the branding of some other newer forms of review does not clearly describe the similarities and differences between these approaches to reviewing; nor does it clarify the potential benefits and limitations of these different methods.

Objectives: To provide a rationale underpinning the evolving range of approaches to reviewing.

Methods: A conceptual analysis of the range of review methods, the dimensions on which they vary and their benefits and limitations.

Results: Reviews vary in terms of: (1) aims and approaches (what the review aims to achieve, the theoretical and ideological assumptions, the use of theory and the balance between aggregating and configuring data); (2) structure and components (the number and type of mapping and synthesis components and how they relate); and (3) breadth and depth and extent of ‘work done’ in addressing a research issue (the breadth of review questions and detail with which they are addressed, and the amount the review progresses a research agenda).

Conclusions: Clarity of the dimensions on which reviews vary, and the strength and weaknesses of the different methods, are required to progress the development and appropriate use of methods for systematic reviewing.