This collective work provides a chronological and up-to-date reconstruction of the three-round debate between Robert Alexy and Ralf Poscher.The debate represents the German development of an enduring jurisprudential controversy over the concept and adjudicatory role of legal principles, classically addressed by HLA Hart and Ronald Dworkin. Alexy’s principles theory, which has initially defined ‘legal principles’ as optimisation requirements, currently argues that they express an ‘ideal ought’. Poscher’s critique challenges the soundness of Alexy’s principles theory by questioning its ontological and epistemological commitments. As legal principles are directly related to constitutional rights, the Alexy-Poscher debate has significant implications for constitutional adjudication. For instance, proportionality analysis – which incorporates a highly controversial balancing test – and the legitimate boundaries of judicial power, hinge on the debate's two opposing views. Yet despite the centrality and pervasiveness of this topic, German contributions to the theoretical and practical impact of legal principles remain generally overlooked by English-speaking scholars. Concluding with David Duarte’s critical and meticulous assessment of the debate, this collection bridges that important scholarly gap. Whether or not conversant in the debate on legal principles, legal researchers and advanced law students with interdisciplinary interests in jurisprudence and constitutional law will find in this book a timely and distinctive introduction to leading developments in German legal thinking.