Pediatricians aspire to optimize overall health and development, but there are no comprehensive measures of well-being to guide pediatric primary care redesign. The objective of this article is to describe the Cincinnati Kids Thrive at 5 outcome measure, along with a set of more proximal outcome and process measures, designed to drive system improvement over several years. In this article, we describe a composite measure of "thriving" at age 66 months, using primary care data from the electronic health record. Thriving is defined as immunizations up-to-date, healthy BMI, free of dental pain, normal or corrected vision, normal or corrected hearing, and on track for communication, literacy, and social-emotional milestones. We discuss key considerations and tradeoffs in developing the measure. We then summarize insights from applying this measure to 9544 patients over 3 years. Baseline rates of thriving were 13% when including all patients and 31% when including only patients with complete data available. Interpretation of results was complicated by missing data in 50% of patients and nonindependent success rates among bundle components. There was considerable enthusiasm among other practices and sectors to learn with us and to measure system performance using time-linked trajectories. We learned to present our data in ways that balanced aspirational long-term or multidisciplinary goal-setting with more easily attainable short-term aims. On the basis of our experience with the Thrive at 5 measure, we discuss future directions and place a broader call to action for pediatricians, researchers, policy makers, and communities. The first 5 years of life are critical in establishing trajectories for lifelong health. 1,2 Cognitive, language, and social-emotional development in early childhood are foundational to entering kindergarten with skills for educational success. 3 Educational achievement, in turn, is linked with health throughout the lifecourse. 4 Health care is the only institution with near-universal access to children in the first 5 years. Accordingly, pediatrics has embraced a role in promoting overall health and well-being to ensure school readiness. 5-8 Well-child care guidelines are comprehensive, expecting pediatricians to provide immunizations; address nutrition, development, behavior, vision, and hearing; and link families with dentists. 9-11 American Academy of Pediatrics policy statements call for pediatricians to assume a larger role in kindergarten readiness, promote highquality early childhood education (ECE), and mitigate the effects of toxic stress and poverty. 6,7,12 Despite offering these wide-ranging guidelines, the literature has not included a framework for measuring pediatricians' effectiveness in efforts to comprehensively promote well-being.