Citing contemporary issues, this introduction situates horizontal application as one potential response to political-legal questions involving private actors. It makes the case for renewed scholarly attention to horizontal application as an increasingly common practice in constitutional politics. More specifically, examining horizontal application through the lens of republican political theory uncovers new significance in the discourses surrounding this constitutional practice. This theoretical perspective also elucidates how horizontal application is different from traditional constitutional understandings. After introducing the book’s republican framework, Chapter 1 explains the rationale underlying the choice of contexts examined in subsequent chapters. It concludes with an explanation of the stakes, as well as the potential benefits and drawbacks of horizontal application considered in the following chapters. Finally, it previews the concluding chapter’s argument that horizontal application may be further supported with certain political and institutional adjustments to make this practice even more republican.