During the last decade evidence-based medicine (EBM) has become a dominant approach in many medical fields, including ophthalmology. Clinical epidemiological studies provide evidence that can aid the decision-making processes. An overwhelming amount of clinical epidemiological papers are being published every year, and critical appraisal of the findings can be challenging, especially for the busy clinician who is not formally trained in the field of clinical epidemiology. Therefore the available evidence is increasingly bundled in clinical guidelines. The aim of this chapter is to provide the readers with some basic knowledge to allow them to judge the value of clinical epidemiological papers and thus of the pillars of evidence-based clinical guidelines. Examples from ocular oncology will be used to illustrate the methodological principles.