Meta-analysis is a statistical analysis of data combined from two or more studies. Different studies use different variables measured on different scales. Meta-analysis is a method of standardizing the studies so that results can be more easily compared. Meta-analysis is a useful tool for evidence-based medicine. Articles in this issue examine the strengths and limitations of meta-analysis in infectious diseases, the use of meta-analysis in comparing diagnostic tests or treatments, and meta-analysis for specific problems, such as parasitic infections, HIV, surgical infections, and pediatric infections.