Default reasoning can be described as the art of drawing plausible conclusions from incomplete or contradictory information. Such conclusions can be classified as being associative, yielding properties readily implied by the given information and background knowledge, or reflective, yielding explanations for abnormalities detected in the given evidence. This book presents a novel mathematical model of how the overall knowledge underlying the process may be represented and organized in a reasoning system and how both types of default conclusions may be formed from that body of knowledge. Rather than focusing on a final state of belief ultimately formed by a given state of knowledge, as in the traditional approaches, the reasoner's belief state is modelled as undergoing a continuing evolution in response to a continuing evolution of the reasoner's knowledge state. This evolutionary perspective allows for monitoring the ongoing epistemic behaviour of reactive reasoners, formulating advanced introspective defaults, detecting and correcting inferential irregularities, and accounting for informational changes that occur during inference.