This article makes first a critical review of the different definitions in the scientific literature of the 'underclass' concept. It is argued that the American and British concept of underclass is not necessarily transportable to Europe. The next part adresses the question of a possible emerging urban underclass in Belgium. It is concluded that although Belgium has to fight against social problems associated with underclass formation in the US and Britain, it has until now no clearcut underclass. The main reasons for this are that economic marginality is not permanent and stable, and that economic marginality, deviant values and criminal behaviour are not systematically coinciding.