Over the last decades, the number of farms has been decreasing and their size increasing all over the European Union. This paper aims at studying the development of agricultural production concentration across EU Member States over the 2004-2013 period. Using an adapted version of the Herfindahl Hirschman Index (HHI), it shows that a variety of situations exists across Europe as regards the distribution of commercial farm sizes measured in Euros of standard output. Results document how the overall tendency to concentration may originate either from the reduction in farm numbers, or from farm sizes becoming more unequal, or both.