Health impact assessment (HIA), a systematic assessment of potential health impacts of proposed public polices, programs, and projects, offers a means to advance population health by bringing public health research to bear on questions of public policy. The United States has been slow to adopt HIA, but considerable strides have been made in many other countries, and under the auspices of the World Health Organization and World Bank. Varied applications in these diverse milieu have given rise to diverse approaches to HIA-quantitative/analytic, participatory, and procedural-each with distinct disciplinary foundations, goals, and methodologies. Suitability of these approaches for different applications and their challenges are highlighted, along with areas in which methodologic work is most needed and most likely to advance the field from theory and infrequent application to more routine practice in the United States.