In this article, we use a large database of more than 3700 interfirm relationships in force between French firms in 2003 to study the way they achieve cooperation. We find that cooperation is enhanced by a partner's reputation. Our results also suggest that while formal contracts have a direct negative impact on ex-post cooperation, they are an enhancing factor of cooperative behaviors as soon as the intrinsic hazards of the relationship require higher ex-ante contractual safeguards to secure agreements.