Systematic reviews use explicit and reproducible criteria to assemble, appraise, and combine articles with a minimum of bias. Meta-analysis is a form of systematic review that uses statistical techniques to derive quantitative estimates of the magnitude of treatment effects and their associated precision. Valid meta-analyses address focused questions, use appropriate criteria to select articles, assess the quality and combinability of articles, provide graphic and numeric summaries, consider potential biases, and can be generalized to a meaningful target population. The rate difference, or absolute risk reduction, is the preferred measure of clinical effect size; the reciprocal tells the number needed to treat for one additional favorable outcome. The benefits of meta-analysis over individual trials include greater precision, increased statistical power, and the ability to identify and explore diversity among studies. Threats to validity include heterogeneity, citation bias, publication bias, language bias, and variations in study quality. Because meta-analysis defines rational treatment expectations at a population level, it is an adjunct to, not a substitute for, clinical judgment in the care of individual patients.