Private organizations play a growing role in governing global issues alongside traditional public actors such as states, international organizations, and subnational governments. What do we know about how private authority and public policy interact? What are the implications of answering this question for understanding support for, and effects of, policy development generally? The purpose of this article is to reflect on these questions by introducing, and reviewing, a special issue that challenges explicit claims, and implicit methodologies, that treat private and public governance realms as distinct and/or static. We do so by advancing a theoretical and conceptual framework with which to explore how the contributions to this special issue enhance an understanding about governance interactions across a range of empirical, sectoral, and regional domains. We specifically introduce the concept of governance spheres to capture the proliferation of issue domains denoted by highly fluid interactions across public and private governance boundaries.