This special issue highlights some of the papers presented at the second annual Active Living Research (ALR) Conference in February 2005. Each represents an important building block in developing a comprehensive knowledge base concerning environmental and policy influences on physical activity. In the summer of 2000 when the Robert Wood Johnson Foundation (RWJF) Board of Trustees approved the concept paper for what would become Active Living Research, none of us could have imagined how quickly this evidence base would emerge. The program staff of RWJF was charged with developing grantmaking strategies that would lead to better approaches to increase population physical activity levels. The 1996 Surgeon General’s report on physical activity had clearly demonstrated the health benefits of exercise, yet health educators, exercise physiologists, and others had shown limited impact in significantly increasing the number of people engaging in even moderate levels of physical activity. 1 RWJF was eager to stimulate substantial improvements in a behavior that had such potential for improving the health of all Americans, as they had with smoking. Physical activity was determined to be an appropriate and worthy target. Much has been written about using ecological models to promote health behavior change, and the benefits of targeting environmental and policy solutions to change population level behavior are generally accepted. 2-7 However, few foundations have invested in developing and evaluating these types of interventions. RWJF has been unique in its understanding of environmental action strategies and its willingness to support policy research to inform those approaches. (See RWJF initiatives such as A Matter of Degree, Reducing Underage Drinking Through Coalitions, Smokeless States and the Substance Abuse Policy Research Program, for example.) As the first step in creating a social change strategy designed to increase population physical activity levels, Active Living Research was funded to investigate which environments and what policies would have the greatest potential to impact physical activity. Initial studies focused on developing and validating measures of the built environment for use in establishing a systematic evidence base. Measurement studies were followed by correlational studies to help determine the relationships between the environment and physical activity levels. Some of the papers in this special issue report on these initial measurement and correlation studies.