Ongoing public debate about the role of science in policy making signifies the importance of advancing theory and practice in the field. Indeed, assumptions about the science–policy nexus hold direct implications for how this interface is managed. A useful lens on contemporary themes is offered by the experience of a federal environmental science program that launched an ambitious effort to enhance capacity for policy relevance while protecting a commitment to sound, impartial scientific inquiry. This was achieved by developing an explicit conceptual model and implementing corresponding strategies that addressed critical gaps in capacity for policy-relevant research, analysis, and communication while supporting existing capacities. This article describes and evaluates the capacity-building effort from the dual perspectives of deepening an understanding of successful practice in the field and advancing a conceptual understanding of the science–policy nexus. It illustrates the challenges facing practitioners and the need for greater interaction between theory and practice.