This article covers the basic principles of systematic reviews and meta-analyses while discussing the problems associated with the traditional narrative review. It explores the role of the overview as a formal method of combining information from all relevant studies about a particular health condition. It also describes how the review tries to minimise bias so as to avoid reaching the wrong conclusions from the available research data. Important issues that need to be considered when appraising an overview are outlined. Some terms used in reporting systematic reviews and meta-analyses are introduced, such as risk ratio, mean difference, confidence interval and the forest plot.