There is an on-going debate in the literature as to whether national parliaments can and do play an active role in EU policy-making. The main reason for persistent disagreement is the lack of comparative empirical data on parliamentary behaviour in EU affairs. The article aims to contribute to this debate by presenting the first comparative quantitative data on European affairs activities of national parliaments and by explaining the empirical variation. The development of a unique dataset including all 27 national parliaments allows a series of explanatory variables to be tested for the level of parliamentary activity at both the committee and the plenary levels. The analysis shows that institutional strength in EU affairs plays an important role. Overall, however, EU activities can be better explained with a mix of institutional capacities and motivational incentives. The specific combinations vary for different types of activities.