Causality is discussed from two complementary points of view : theoretical and empirical. The theoretical discussion attacks the position adopted by Hume that there is no such thing as causality. It takes ideas from Piaget and from Cognitive Science to present an analysis of causality in which action and concrete models are important. This view is supported by arguments from linguistics. Several empirical studies of causal reasoning relevant to science education are described. One tests a causal theory of everyday ideas about force and motion. The others explore the ontology attribute by subjects to a number of entities, including everyday entities and others of interest in science. Some fundamental ontological dimensions are proposed.La ca...