The paper aims at analysing the dimension of the underground economy in the EU states between 2003 and 2015. In order to achieve this goal, we use qualitative and quantitative methods. In the first two sections of the study we resort to conceptualisation and bring up to date scientific contributions in the field of the shadow economy both in terms of its definition and the methods of quantifying the phenomenon. In the third section, based on the results of the latest research, we identify the dynamics of the underground economy on three levels: the EU-15 (before and after 2005); the EU-10 (before and after the accession of 10 countries in 2005) and the EU-3 (before and after 2007). The main conclusion is that regardless of the reporting level (national, European, world), the real size of the underground economy remains unknown.