In this article illegal immigrants, a relatively new group of immigrants living at the margins of society, are discussed. The question of the significance of crime for groups that are officially excluded from the formal labour market and public provisions, is presented within the framework of the Unknown City research project, conducted in the four largest Dutch cities (interviews with illegal immigrants; an ethnographic study to determine the extent of support by various ethnic communities; and an examination of the ways in which the restrictive policies towards illegal immigrants were implemented by the police, the Aliens Departments, and by professionals within public or semi-public institutions in the fields of education, healthcare and housing). Both the relatively limited involvement in crime in general and the differences between groups can be explained by the embeddedness of illegal immigrants in the labour sphere and the support by ethnic communities. Attention is paid to the social and legal construction of the illegal immigrant through new legislation and to the observation that illegality is increasingly linked to crime. The majority of illegal immigrants are not criminally active. One exception is the category that is active in the lower levels of the hard drug trade. The authors' analysis suggests that the perception of the 'criminal illegal immigrant' first and foremost reflects the division between wanted and unwanted immigrants, which is the result of the shift towards a restrictive policy.