This Article argues that inter-agency coordination is one of the great challenges of modern governance. It explains why lawmakers frequently assign overlapping and fragmented delegations that require agencies to “share regulatory space,” why these delegations are so pervasive and stubborn, and why consolidating or eliminating agency functions will not solve the problems they create. The Article describes a variety of tools that Congress, the President and the agencies can use to manage coordination challenges effectively, including agency interaction requirements, formal inter-agency agreements, and joint policymaking. The Article assesses the relative costs and benefits of these coordination tools, using the normative criteria of efficiency, effectiveness and accountability, and concludes that the benefits of coordination will frequently justify its costs. To varying extents, these instruments can reduce regulatory costs for both government and the private sector, improve expertise, and ameliorate the risk of bureaucratic drift without compromising transparency. Coordination can also help to preserve the functional aspects of shared or overlapping authority, which include promoting inter-agency competition and accountability, while minimizing its dysfunctions in terms of discordant policy. While burdensome, shared regulatory space should also provide an important opportunity for the President to extend his reach. The Article argues that the President is uniquely positioned and motivated to manage the problems of shared regulatory space, and that coordination tools afford him the chance to put his stamp on policy. The Article recommends a comprehensive executive branch effort to promote stronger inter-agency coordination and improve coordination instruments. Of course, any presidential exercise of centralized supervision must operate within legal bounds, and often will be politically contentious. On balance, however, presidential leadership will be crucial to managing the serious coordination challenges presented by modern governance, and existing political and legal checks on potential overreach are sufficient to manage any conflicts with Congress. The Article concludes by exploring the implications of enhanced inter-agency coordination for judicial review. Courts might adjust standards of review to promote coordination, but even under existing standards of review policy decisions arrived at through strong inter-agency coordination likely will attract greater deference. The Article shows that greater coordination is relatively unlikely to impact the outcome of the Chevron inquiry for reviewing agency legal interpretations. Yet it also suggests some minor doctrinal adjustments that could lead to greater deference where agencies use certain coordination instruments to adopt shared legal interpretations. The larger conceptual purpose of the Article is to draw attention to the phenomenon of shared regulatory space and highlight the pressing need for inter-agency coordination as a response. It invites scholars and practitioners to focus on inter-agency dynamics, which requires a departure from the single-agency focus that has traditionally been so central to administrative law.