The comparative international Human Resource Management (HRM) literature presents competing perspectives as to why HRM looks different in different countries. This paper takes a new approach to exploring relevant drivers behind HRM in European countries by looking simultaneously at three suggested sources of influence: national culture, national institutions, and a supranational institution. To date, national culture and institutions have frequently been used to explore HRM drivers, but rarely simultaneously, whilst the effect of the European Union (EU) Social Charter has received much less attention. Here, countries are clustered according to these three sources of influence, and survey evidence from 17 EU countries is used to explore the extent of difference and similarity in HRM practices within these clusters. Results indicate that all three variables show patterns in HRM practices; however, they only explain relatively small amounts of variance. National-level institutional factors appear to have the strongest influence.