Considerable research illuminates the development of causal understanding. However, the research base is hardly a coherent whole. Some is based in research on children's understanding of particular science concepts. Some grows out of social psychology and considers how one attributes intentions and behaviors. Some comes from the developmental research and attempts to answer key questions that lend insight into the nature of one's mind--such as whether the understanding of causality is innate or learned. This article draws from across these diverse literatures to sketch the current knowledge of what causal understanding looks like and how it develops in relation to the development of scientific understanding. The discussion here is framed by a taxonomy created with the author's colleague, David Perkins (Perkins & Grotzer, 2000), in the hope that it will begin to illuminate how various aspects of causality relate to one another. Differences in terminology and techniques within the research can lead to, in some instances, contradictory outcomes. This article aims to be explicit about these differences and to rely upon those findings for which a fair amount of consensus emerges or where the discrepancies have been adequately addressed. The research presented defines and attempts to answer some key questions related to the development of causal understanding and its relationship to science learning.(Contains 1 table and 22 notes.)