General considerations about meta-analysis and the different steps of this technique are successively discussed: definition of the main objective, identification of the outcome, description of the retrieval and selection of trials, description of the statistical analysis and interpretation of the results. Advantages and drawbacks of the meta-analytical technique are then described: 1) scientific approach, possible quantification of the therapeutic effect, increase of the power of a future clinical trial, synthesis of contradicting results, assessment of the homogeneity, subgroup analysis, analysis of sensibility, scientific collaboration, help for therapeutic information. 2) retrospective approach, inconsistency among trials, potential biases, persistence of some unsolved methodological problems, difficulties for a critical reading and for the interpretation of conclusions. In addition, some examples of published meta-analyses are given to illustrate the advantages and limits mentioned above.