The application of European Union (EU) rules is, in general, the responsibility of national executives. This key intergovernmental aspect of the EU's administrative order makes compliance with supranational law vulnerable to distortion. However, the European executive has added important fire-alarm oversight mechanisms by means of transgovernmental networks (TGNs) to its toolbox. This article examines the work mode, horizontalness and effectiveness of such networks as newer governance tools to oversee and monitor the compliance with EU law. It draws on a unique dataset on the Solvit network, enabling us to examine effectiveness and variation of a transgovernmental network in operation. The article substantiates the relevance of TGNs in identifying and solving manifold and complex problems of misapplied EU law, finds that the Commission constitutes a focal point in this type of multilevel executive and points out that learning in part explains why effectiveness varies across member states.