A number of factors may motivate authors to undertake a systematic review. For example, reviews can be conducted in an effort to resolve conflicting evidence, to address questions where clinical practice is uncertain, to explore variations in practice, to confirm the appropriateness of current practice or to highlight a need for future research. The overarching aim of Cochrane reviews should be to summarize and help people to understand the evidence. They should help people make practical decisions about health care. This aim has important implications for deciding whether or not to undertake a Cochrane review, how to formulate the question that a review will address, how to develop eligibility criteria to guide study inclusion based on the review question, how to develop the protocol and how to present the results of the review.