Background&Aims: In recent years there has been considerable growth in the public health research area investigating the influence of the built environment on physical activity. Simultaneously, environmental, transport and planning professionals have been promoting a modal change to active transport modes to reduce traffic congestion and air pollution. A core factor in both areas of research is the concept of ‘walkability’. International literature suggests that walkable areas are favourable to active transport (i.e., walking and cycling) but the factors used to identify what a walkable area or neighbourhood actually is can be varied and contradictory. In addition, many professionals and policy-makers have different ideas or expectations as to what makes an area walkable or not. The purpose of this paper is to examine how various professionals and policy-makers in Dublin understand and operationalize the concept of walkability. A key component of this study is to understand to what extent these decision-makers consider public health concerns as well as others concerns such as safety issues, traffic management, air quality, the nature of the local economy, and aesthetics, among others. Methods: This study is part of a larger study called the The ‘Cleaner, Greener, Leaner Study’, which is funded in part by the Irish Environmental Protection Agency’s STRIVE programme. The study aims to examine to what extent urban form impacts individual level health; transport behaviour; and hence the production of air pollution and greenhouse emissions in the Greater Dublin Area. The first stage of the study is to develop a local definition of ‘walkability’ that is suitable for use in an Irish context. In order to understand the views of various relevant professions on the concept of ‘walkability’, we conducted a comprehensive survey targeting professionals and policymakers who work in the area of planning, urban design, engineering, and construction of the built environment and transport systems. Our final sample size was 218 professionals and policy-makers. In addition we conducted five in-depth focus groups with a sub-sample of these decision-makers. We use information from these focus groups to further interpret our quantitative based survey findings Results: The data collection phase for this stage of the project was recently completed. We are in the process of analyzing our findings. Initial results, clearly indicate that whilst quality footpaths or sidewalks and nearby destinations are important to all professional groups and policy-makers there are some significant differences on how professionals view walkability and its value to the public. Many professionals and policy-makes do not adequately consider health concerns when making decisions about the nature of the built environment. Conclusions: We feel our study is important because we work to “get inside the heads” of professionals and policymakers who make decisions about the way our built environments function and the way we travel. In doing so, we provide key insights into the conceptualization of walkability and the thinking that affects decisions to build or not build places that enable active transport and improve health.