This chapter examines the origins of causal understanding. It analyses studies where causal reasoning characterizes the basic causal mechanisms in terms of which people explain the world around them. It considers two distinct views of cognitive modules in the analysis of causal understanding: first, they are seen as analogous to input modules, and second, they are seen as intuitive theories. On both views, cognitive modules pick out a set of entities in the world and are responsible for processing privileged sorts of information about those entities.