Mending the hole in multilevel implementation: Administrative cooperation related to worker mobility

European economic integration creates unintended consequences for national public administrations. This article offers a conceptual and empirical analysis of how these challenges are met. First, three challenges are identified: a reduced capacity to offer services to citizens who move freely, increasing administrative burdens, and negative externalities for all parts from a single states' administrative failure. Second, a conceptual framework is developed that links each challenge to a most likely response in form of modes of administrative cooperation. Third, the framework is illustrated by an empirical analysis of the coordination of social security systems, labor inspectorates, and posted workers. The case studies show that horizontal administrative cooperation is developed stepwise over time and in line with the theoretical framework. In sum, we can sustain that horizontal administrative cooperation is a relevant additional integration dynamic that buffers unintended effects of market integration on formally independent but increasingly interdependent member state executive bodies.