After its implementation in 2011, the European Semester has become a much studied governance model. The Semester brings together existing EU socio-economic coordination cycles and introduces new coordination mechanisms, using the rich knowledge the EU has gained in decades of policy coordination. Although building on long-standing structures, the Semester may also be seen as a major step in EU governance, or even as a coordination innovation. This chapter first describes the history of EU socio-economic coordination prior to the Semester. Second, it explains how the European Semester operates, including its complexities stemming from bringing different coordination mechanisms together, and from the dynamic interactions of actors across national and European governance levels. Third, it outlines why the European Semester may be seen as innovative, and gives theoretical and methodological suggestions on how to study the Semester.