The chapter explores how Anthony Giddens’s theory of structuration can inform the study of practices in international law. The chapter suggests that rules are implicated in practices in three ways: they constitute patterns of action as ‘practices’, regulate the conduct that makes up a practice, and provide formulas for extending and adapting the practice to ever new situations. At the same time, the rules instantiated in practices are potentially transformed by those very practices. To capture this mutually constitutive relationship between rules and practices, the chapter develops a definition of practices as simultaneously rule-generated and rule-generative patterns of action. The chapter then employs this definition to explore the relationship between international law and practices. Since the practices of a wide array of actors are involved in the constitution of international law, the chapter next discusses the responsibility of those actors whose practices can have an impact on what international law is, focusing on the role of academics. The chapter concludes by outlining three challenges that empirical research of practices confronts: accessing information about practices, analytically separating the effects of agency and structure, and developing research designs which do not simply overlay familiar debates with a practice vocabulary.