 Reasoning is defeasible when the corresponding argument is rationally compelling but not deductively valid. The truth of the premises of a good defeasible argument provide support for the conclusion, even though it is possible for the premises to be true and the conclusion false. In other words, the relationship of support between premises and conclusion is a tentative one, potentially defeated by additional information. Philosophers have studied the nature of defeasible reasoning since Aristotle’s analysis of dialectical reasoning in the Topics and the Posterior Analytics, but the subject has been studied with unique intensity over the last forty years, largely due to the interest it attracted from the artificial intelligence movement in computer science. There have been two approaches to the study of reasoning: treating it either as a branch of epistemology (the study of knowledge) or as a branch of logic. In recent work, the term defeasible reasoning has typically been limited to inferences involving rough-and-ready, exception-permitting generalizations, that is, inferring what has or will happen on the basis of what normally happens. This narrower sense of defeasible reasoning, which will be the subject of this article, excludes from the topic the study of other forms of non-deductive reasoning, including inference to the best explanation, abduction, analogical reasoning, and scientific induction. This exclusion is to some extent artificial, but it reflects the fact that the formal study of these other forms of non-deductive reasoning remains quite rudimentary. 