Scientific and medical information is nowadays abundant and easily available because of advances in information systems and changes in the quantity of research. This multiplicity requires a synthesis if one wishes to find a specific answer to a question. Meta-analysis is a method to perform such work by combining the results of several studies to produce an objective synthesis according to a specific, reproducible protocol. It also allows one to quantify the overall result for all studies considered, and thus obtain a more precise and more acceptable generalization. Meta-analyses of summary data presented here, as opposed to those based on individual data, are more frequently used because they make it easier to aim for exhaustivity, essential for an unbiased synthesis. The work of synthesis should be detailed in a protocol to ensure reproducibility. It occurs in four stages: research studies, selection, data extraction, and analysis. Its feasibility is mainly limited to the existence of sufficient numbers of studies that may be included. This method is widely used in medical research since the 1970s, as well as care research. It has become essential for the development of recommendations. The scope here is limited to that of previous studies. The evaluation of the effect of drugs and other therapies, as well as epidemiology, screening, diagnosis, and nursing research, are areas where meta-analysis can be applied.