Healthy people live in healthy communities. Nowhere is that more apparent or more challenging than in cities. Cities face unique challenges with regard to changes in their economic base, concentrated poverty, housing quality and affordability, violence, and pollution exposure. Health outcomes closely track with these challenges.1 The health of cities is often expressed in aggregate measures of the health of individuals and in terms of specific diseases, injuries, and causes of death. The implication is that if individuals simply reduced risk factors such as poor diets and had better access to medical care, they could reach their full potential for health. However, that is a gross oversimplification. Health is much more a product of the social and environmental conditions in which individuals live, work, and play that influence health directly and indirectly. By one estimate, for example, approximately 20% of health can be attributed to access to medical care, 30% can be attributed to health behaviors such as smoking and exercise, 40% attributed to socioeconomic factors such as employment and income, and 10% to the built and natural environments including influences such as air quality and access to safe places to exercise.2 In a recent survey, big city health officials identified “health in all policies” (HiAP) as a top priority.3 The concept of HiAP has international roots. The World Health Organization's (WHO) definition of health as “a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity”4 underscores the importance of a broad framework for understanding the factors that contribute to the health of individuals and populations. Beginning in 1988, the WHO has advocated for “healthy public policy,” which focuses on working across sectors and beyond the medical care system to ensure that health is taken into account when forming public policy.5 Building on that work, in 2006 Finland adopted HiAP as its main health theme during its presidency of the European Union, with the goal that European Union and Member States' policies would begin to take health impacts into consideration.6 The WHO defines HiAP as: An approach to public policies across sectors that systematically takes into account the health and health-system implications of decisions, seeks synergies, and avoids harmful health impacts, in order to improve population health and health equity. The HiAP approach is founded on health-related rights and obligations. It emphasizes the consequences of public policies on health determinants, and aims to improve the accountability of policy-makers for health impacts at all levels of policy-making.7 Since 2006, HiAP has quickly gained momentum in the United States and internationally.8,9 In recent decades, the link between social and economic factors—often termed “social determinants”—and health has become firmly established in research. The social gradient, whether measured by social class, income, or education, is linked to life expectancy, quality of life, and many other specific health outcomes from injuries to mental health and cardiovascular disease.10,11 Neighborhood and housing conditions play a critical role in a wide range of illnesses, and education—particularly in early childhood—is among the most important factors determining one's lifelong health.