The author of this Policy Brief, Pascale Vielle, Lecturer at the University of Louvain, identifies the potential benefits and likely pitfalls associated with the Horizontal Social Clause in Article 9 of the Treaty on the Functioning of the European Union in the light of the experience of the gender mainstreaming clause. She shows that only a firm commitment on the part of all relevant European actors can ensure the successful development of horizontal policies on the scale of the EU.