This report investigates the utility of abstraction for agents living in complex, dynamic environments. The generation of intelligent behavior in such environments requires the integration of deliberative and reactive processes. Modularity and hierarchy have proven to be valuable organizational principles in this context, and the notion of "levels of abstraction" has played a particularly important role. This report presents a form of abstraction called supervenience, of which other common forms of abstraction are special cases. Supervenience is based on epistemological "distance from the world," and is particularly useful for integrating deliberative processes with actions in a changing environment. Supervenience is discussed in relation to the literature of AI planning systems, the literature of cognitive psychology, and the philosophical literature in which the term originated. Supervenience is described in the context of nonmonotonic reasoning systems, and is compared to related formal constructs. A program based on the concept of supervenience is described, and its performance in a dynamic-world planning domain is demonstrated. This report is a reformatted version of the author's doctoral dissertation.