The human response to an environmental hazard can either reduce or exacerbate its impact on health. This article reviews determinants of community-level responses to environmental health hazards. The aim is to identify factors that can enhance a community's capacity to protect itself and to suggest public health strategies that can increase such capacity. Four case histories of community environmental health action are presented to test a theoretical model for understanding the determinants of community capacity to promote environmental health. Specific actions public health professionals can take to strengthen community capacity include increasing access to accurate science, building strong relationships between communities and local health departments, and supporting political reforms that level the playing field for communities that seek to challenge corporate or government practices. (Contains 3 tables and 1 figure.)