The development of evidence-based medicine has encouraged clinicians to consider the best available evidence when making clinical decisions, individualised for the patient.1,2 A hierarchy of levels of evidence about therapy is described, with the highest being meta-analyses and large, well-designed randomised controlled trials.3-5 Lower down the hierarchy are observational studies, a group that includes cohort, case-control and cross-sectional studies. Here we describe the benefits and limitations of observational studies, and how to assess their quality.