The systematic literature review is a powerful tool for summarizing and evaluating current knowledge related to a specific research question. Systematic reviews have many advantages over traditional narrative reviews. A meta-analysis of data from a systematic review can provide a better estimate of a treatment effect than can individual studies. To ensure quality conclusions, rigorous methods must be applied to systematic reviews, such as formulation of a specific research question, systematic literature search, selection and assessment of included studies, data extraction, quality assessment of included studies, meta-analysis and presentation of results, and formation of conclusions. Threats to the internal validity and generalizability of the conclusions of systematic reviews include lack of clarity or appropriateness of the research question, poor quality of the included studies, heterogeneity of results between studies, inappropriate conclusions, and inappropriate application in clinical practice.