Enterprise ontology is one of the conceptual pillars of enterprise engineering, next to enterprise design and enterprise governance, together accomplishing the goals of intellectual manageability, organisational concinnity and social devotion. By revealing the essence of an enterprise’s organisation, enterprise ontology addresses business processes, data and rules in a fundamental and truly integrated way. In addition, it provides deep insight into and broad overview over complex organisational transformations. The book is divided into three parts. Part I is an introduction in enterprise engineering and enterprise ontology. Part II explores the theories underlying enterprise ontology, explaining the foundations of each theory, the elaborations in practical methods and techniques, and the relationships with other comparable approaches. Part III presents the practical application of the theories. It includes a comprehensive summary of the DEMO methodology and the DEMO specification language, as well as exercises and applications of DEMO in various business areas. It also features a chapter on combining DEMO with comparable approaches to modelling business processes, data and rules, to the benefit of the latter. This second edition comprises two major improvements, driven by increased theoretical precision and further practical experience with DEMO. One is the clear separation between documents or data sets and the files that carry them, the other one is the re-positioning of the input data in the action rules. Discussing the theoretical foundations of enterprise ontology and its practical applications in equal measure, this book is the principal textbook in courses on enterprise engineering. Since it unites elements from management science and information systems engineering, it is also relevant to students and professionals in either field.