The database provides an inventory and a critical appraisal of data and estimates in the European Union and in selected member states. It contains estimates on the size of irregular migrant populations and indicators of their composition with regard to gender, age, nationality and sector of economic activity. The presentation is innovative in its consistent structuring and its carefully developed quality classification, which indicates whether estimates are more or less trustworthy. The summarizing tables are designed to give users the best possible overview of quantitative data in the countries, in a simplified form. The researchers involved in the creation of this database are aware that irregular migration is a complex issue. Therefore, quantitative information is accompanied by substantial background materials, both on issues of general concern and on the situation in individual countries.