Systematic reviews retrieve, appraise and summarise all the available evidence on a specific health question. They are designed to reduce the effect of the reviewers' own bias, and a full protocol should be written to define and guide the process. The appropriate resources should be in place before undertaking a review. The steps of the review are: frame the question and choose appropriate methods; identify relevant work; extract relevant data on outcomes and quality; summarise the evidence; and, interpret the evidence. Reviews that combine valid, homogeneous studies of treatments that are relevant to health care, in patients who are typical, can provide good evidence to guide health care decisions.