The capability to reason in a world full of uncertainties, vagueness and ignorance is what distinguishes humans. This ability to argument in a partially known world is the informal definition of common-sense reasoning. The question how common-sense reasoning is performed occupied humanity since we can think of. Last century this issue reached an immense importance. Especially during the last three decades the study of common-sense reasoning became one of the major research topics in Artificial Intelligence (AI). Several formalisms to capture the mechanism of common-sense reasoning have been proposed so far. This book concentrates on presenting the most important formalisms for common-sense reasoning, and, showing that one of the discussed formalisms serves perfectly to capture the mechanism of common-sense reasoning, since this formalism subsumes all other in this book introduced formalisms dealing with common-sense reasoning.