Systematic reviews adopt a rigorous scientific approach to identify and synthesize all the available evidence pertaining to a specific research question. They are carried out according to pre-defined protocols, which set out the scope of the systematic review, details of the methodology to be employed and reporting of findings. The objective is to ensure a comprehensive and repeatable search together with an unbiased assessment and presentation of the relevant evidence. Key components of a systematic review include: systematic and extensive searches to identify all the relevant published and unpublished literature; study selection according to pre-defined eligibility criteria; assessment of the quality and risk of bias in included studies; presentation of the findings in an independent and impartial manner and a discussion of the limitations of the evidence and of the review.