This paper describes the inference explanation capabilities of Cyc, a logical reasoning system that includes a huge “commonsense” knowledge base and an inference engine that sup- ports both question answering and hypothesis generation. Cyc allows the user to compose queries by means of English templates, and tries to find answers via deductive reasoning. If deduction is fruitless Cyc resorts to abduction, filling in missing pieces of logical arguments with plausible conjec- tures to obtain provisional answers. Cyc presents its answers and chains of reasoning to the user in English, provides drill- down to external source references whenever possible, and reasons about its own proofs to determine optimal ways of presenting them to the user. When a chain of reasoning relies on conjectures introduced via abduction, the user can interact with the inference explanation to confirm or deny the ab- duced supports. These capabilities are grounded in the integration of Cyc’s natural language components with the knowledge base and inference engine, and in Cyc’s capacity to maintain an explicit in-memory record of the facts, rules, and calculations used to produce successful proofs during in- ference.