The importance to public health of environmental decisionsincluding those about land use, transportation, power generation, agriculture, and environmental regulation-is increasingly well documented. Yet many decision makers in fields not traditionally focused on health continue to pay little if any attention to the important health effects of their work. This article examines the emerging practice of health impact assessment and offers real-world examples of its effective implementation, including studying the impact of nearby highways-a major source of air pollution-on proposed new housing for seniors. The article argues that officials at the federal, state, and local levels should consult health experts and consider using health impact assessments when their decisions on such issues as urban planning, land use, and environmental regulation have the potential to directly affect the conditions in which people live and work.E nvironmental health, urban planning, and environmental regulation have common origins in efforts to address urban crowding, infectious diseases, and industrial pollution. However, these disciplines have diverged and today generally operate in separate domains. 1,2 Yet research over the past decade has revealed connections between public health and a wide range of environmental factors such as transportation systems, land use, parks and other open space, housing, and energy production. These factors have been linked to most of the leading causes of illness and death in the United States, including obesity, diabetes, asthma, injury, cardiovascular disease, and cancer. [3][4][5] This body of research gives rise to a fundamental challenge for public health: Many important influences on health are determined by decisions made outside the jurisdiction of health agencies and, frequently, without input from public health experts. This challenge is behind an increasingly urgent call for cross-sector approaches to health promotion. 6,7 It is the underlying motivation for recent high-level policy efforts such as California's executive order to include "health in all policies," and the new US cabinet-level National Prevention, Health Promotion, and Public Health Council. 6,8,9 Ultimately, however, successfully engaging government agencies-such as energy and environmental regulators, transportation and landuse planners, and housing departments-in efforts to improve public health will require practical tools for integrating health into the specific procedures that officials use to make decisions, develop regulations, and revise plans. This article focuses on the emerging field of health impact assessment-an approach that is showing early promise as a way to include health considerations in decisions that otherwise would not have taken health into account. 10