Epidemiological observational studies of etiology relate individual characteristics, personal behaviours, environmental conditions, and treatments as ‘exposures’ that may modify risk of disease. These reviews will predominantly include observational studies such as prospective and retrospective cohort studies, case control studies and analytical cross-sectional studies. Randomized controlled trials may also report on the risk associated with an exposure and can be included. Prospective cohort studies usually provide stronger evidence than case-control studies when addressing etiological questions or issues.