There are vexed questions with respect to our understanding of organised crime, particularly around its definition, the classification of the different forms it assumes and the specific functions it performs. This contribution will address them only after providing some material for reflection which derives mainly from two types of sources. The first source is identified by a variety of official organisms investigating, studying and combating organised crime and its activities. These include the United Nations Office on Drugs and Crime (UNODOC), the European Commission, the European Central Bank, Europol and the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA). The second source used in this chapter consists of a range of independent research studies conducted in specific countries. The examination of the material presented will help highlight the dilemmas, the controversies and, indeed, the vexed questions that still adumbrate our understanding of organised crime, while it will pave the way for a tentative classification that can clarify, at least partially, the contours of the subject matter.