© Springer International Publishing Switzerland 2014. 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. Evidence-based medicine and clinical practice guidelines are dependent upon meta-analyses to guide their recommendations. Evidence-based medicine is an apt term to the extent that it advocates more reliance on clinical research than on personal experience or intuition; and, has led to a paradigm outlining the “level of evidence” that addresses a particular clinical question (also see Chap. 3). These “levels of evidence” are also utilized by clinical practice guidelines, but “as the number of available guidelines provided by a variety of sources has literally exploded, serious questions and controversies have arisen about how guidelines should be developed, implemented, and evaluated”.