Sustainable community health improvement often requires the implementation of complex interventions in complex systems. Drawing from the "Four Keys to Success" frame (theory, implementation, support, and evaluation), this article describes how we used a formative evaluation approach to foster a learning system capable of monitoring and addressing emerging community needs within the Spreading Community Accelerators Through Learning and Evaluation (SCALE) initiative--a national capacity-building effort to support 24 community coalitions' progress toward a Culture of Health. The formative evaluation approach resulted in critical advancements to the theory, implementation, and nature of supports provided in SCALE. These improvements enabled the SCALE evaluation team to shift from the initial focus on program implementation issues to a greater emphasis on downstream factors (community-level outcomes). The ability of formative evaluation to grapple with the emerging challenges of implementing complex interventions in complex systems makes it particularly valuable for community health improvement initiatives.