In the health sciences, definitions of cause and effect have not been tightly bound with methods for studying causation. Indeed, many approaches to causal inference provide no definition, leaving users to imagine causality however they prefer. Without a formal definition of causation, an association is distinguished as causal only by having been identified as such based on external and largely contextual considerations. Because they have historical precedence and are still widely used, this entry first reviews such methods. It then discusses definitions and methods based on formal models of causation, especially those based on counterfactuals or potential outcomes.