Undocumented migration in the EU has become a major socioeconomic issue and could burden society and compromise the standards of European cities. The EU-funded project 'Irregular migration: Counting the uncountable. Data and trends across Europe' (Clandestino) helped policymakers articulate policies on the issue. It collected data and numbers on undocumented migration in different countries and analysed the data thoroughly. The countries involved were Czech Republic, Germany, Greece, Spain, France, Italy, Hungary, Netherlands, Austria, Poland, Slovakia, and the United Kingdom. The project also looked at transit countries, notably Morocco, Turkey and the Ukraine, studying the mechanisms and dynamics behind how migrants cross through these countries and into the EU. in addition, Clandestino examined different methods to assess migrant populations and devised a new way to estimate undocumented migration. It then built a database on irregular migration outlining migrant populations in each country and looking at legalisation of undocumented migrants as well. The database reports also investigated relevant policies in every country involved and prepared policy recommendations for each. Policy briefs were also established for the three transit countries. the project undertook a comparative analysis for each country. The analysis covered the size of irregular migration, data collection methods, definition of illegality, control policies and other important topics. Subsequently, Clandestino prepared six research papers on the different aspects of irregular migration. these papers, policy briefs and findings were combined in a book titled 'Irregular migration in Europe: Myths and realities', now available in bookstores. The project also contributed to the journal, International Migration. It organised workshops in Greece and the United Kingdom, in addition to conducting field visits to the countries involved in order to disseminate findings among non-governmental organisations (NGOs), policymakers, think tanks and journalists. Contacts with the European Commission were also undertaken to support the project's objectives. The plethora of information emerging from Clandestino is very likely to shape European policy on migration for years to come.