This chapter gives an overview of theoretical approaches to fragmentation and interaction. Regime interaction can be illustrated and discussed in various thematic settings. Perspectives on regime interaction and the focus of discussion can vary significantly, influencing how a specific issue is addressed in legal theory and in practice. The discussion and evaluation of regime interaction in the law of the sea is not merely an academic exercise. International organizations and policymakers as well as national and international courts and tribunals have to respond to interactions to establish if and how rules and regulations influence one another. Interdisciplinary interfaces are not limited to the interaction between the law of the sea and natural science, although this may be the most relevant relationship in the context of interpretation and application of the United Nations Convention on the Law of the Sea. The chapter also presents an overview of the key concepts discussed in this book.