Meta-analysis refers to methods for the systematic review of a set of individual studies (either from the aggregate data or the individual patient data) with the aim to quantitatively combine their results. This has become a popular approach to attempt to answer questions when the results from individual studies have not been definitive. This chapter will discuss meta-analyses and highlight issues that need critical assessment before the results of the meta-analysis are accepted. Some of these critical issues include: publication bias, sampling bias, and study heterogeneity. © Springer Science + Business Media B.V. 2008.