DESCRIPTION: Concept acquisition, structure and use intimately depend on the larger explanatory systems in which concepts are embedded; yet, there has been little work exploring how such systems of explanation develop and come to influence concepts. Six sets of studies will explore how intuitive explanations and understandings emerge in development and how they are related to notions of cause, mechanism and agency. These issues are then linked to broader questions of what concepts are and how they develop. One set of studies explores how a key aspect of explanatory belief works far above the level of knowledge of specific mechanisms and instead involves knowing what sorts of properties are causally potent in a domain and how they are likely to interact. These patterns vary dramatically across large scale domains of phenomena and a partial understanding of these patterns emerges very early in development and guides learning of more detailed beliefs in domains. A second set of studies show children know that explanations and mechanisms are related even when they know few, if any, details of those explanations and mechanisms. They accomplish this by picking up on abstract relational patterns shared by natural phenomena that might appear on the surface to be quite different. A third set of studies explores how children develop preferences for some novel explanations over others even when there is little or no specific knowledge of the phenomena under explanation. A fourth set of studies examines how emerging knowledge of concrete mechanisms acts a bridge between frequency based information and abstract explanatory principles and how it can distort judgments as well as aid them. A fifth set of studies explores constraints on notions of agency and how limitations on those senses strongly guides explanatory constructs. Finally, a set of studies asks about information that children might use to infer the reliabilities of explanations and how this understanding is related to an emerging appreciation of the necessary division of cognitive labor for detailed explanatory knowledge.