This article presents selected results of a research project on irregular migration and migrants' irregular economic activities in the Czech Republic. The project explored several topics through quantitative as well as qualitative research techniques: a questionnaire survey (N=159) and interviews (N=37) with irregular migrants living in the Czech Republic and a Delphi survey among experts on migration. The article describes and explains the nature of migrants' irregular economic activities. It also provides a quantitative analysis of migrants' selected characteristics followed by a description and typology of relations within the so-called 'client' system.