This book assesses the impact of norms on decision-making. It argues that norms influence choices not by being causes for actions, but by providing reasons. Consequently it approaches the problem via an investigation of the reasoning process in which norms play a decisive role. Kratochwil argues that, depending upon the strictness the guidance norms provide in arriving at a decision, different styles of reasoning with norms can be distinguished. While the focus in this book is largely analytical, the argument is developed through the interpretation of the classic thinkers in international law (Grotius, Vattel, Pufendorf, Rousseau, Hume, Habermas).