The determination of causality is crucial in medicine: for example, a doctor who prescribes a therapeutic regimen believes that it will cure the disease. Causality in medicine is complex, since it can be difficult to establish a cause-effect relationship: arterial hypertension is a known risk factor for stroke, but most people with hypertension will not have a stroke, and most people who have strokes are normotensive. In this article we will define causality, then show how to determine a cause-effect relationship, and finally we will present the type of studies that can provide the strongest evidence on causality.