Does a strong committee system reduce the ability of political parties to dominate a parliament? This article seeks to answer this question in the case of the European Parliament (EP). Specifically, the article assesses the extent to which party leaders control their committee members in the EP. On the basis of interviews with Members of the European Parliament, the article analyses: (1) the extent to which EP party group leaders control committee assignment and (2) how much influence they have over the direction of committee activities, specifically through group co-ordinators. The results show that national delegation leaderships are increasingly involved in directing the committee assignment process and that group co-ordinators, in some cases, are able to control committee business in the EP.