Institutional interaction and complexity are crucial to environmental governance and are quickly becoming dominant themes in the international relations and environmental politics literatures. This chapter lays the conceptual foundations for the volume and provides an overview of its structure and contents. First we introduce four core concepts that provide the common basis for individual contributions and allow investigation of the two central themes of the volume. In this way we establish our understanding of international institutions, institutional interaction, interplay management, and institutional complexes. Thereafter we outline the structure of the book and offer a brief overview of the contents of each chapter.