This chapter covers causality and aetiology. It analyses the various components of causation in medicine: manipulability, comparative form, causal roles vs causal capacities, plurality of causes and probabilistic causality, and the formal logic of counterfactual dependence. There are neither necessary nor sufficient causes in medicine, but tendencies toward necessity or tendencies toward sufficiency which may be quantified by using epidemiologic methods. What we call causes in medicine are only partial, contributive causes. Causation is epistemic and is not something in the world since causes are like inference tickets. Causation involves concepts of various types of incidence rates, and of risks. Even though, philosophically, causation is a plural concept which dissolves in the medical evidence for it, the nature of causation in medicine is not descriptive but intentional and it is linked to effective strategies, to prevention, treatment, and public health policy.