Reasoning and decision making are fundamental parts of the knowledge representation and reasoning (KR&R) artificial intelligence (AI) approach. Research on the two topics of reasoning and decision making is often done in isolation, with different methods and different theoretical understandings for the two topics. The chapter distinguishes research along representation lines, taking particular aim at logic-based and probability-based representations. Research on representation languages that permit tractable query answering yielded specialized languages with large bodies of applications. The chapter describes work along these research paths, focusing on logical reasoning, probabilistic reasoning, and commonsense reasoning. Decision making as a research area spans the disciplines of economics, psychology, computer science, and virtually all the engineering disciplines. The chapter looks at approaches to autonomous decision making developed over the past fifty years. Research on commonsense reasoning is divided into three main streams: logical theory, large commonsense knowledge bases, and ad hoc commonsense reasoning techniques.