The multitude of quality of life problems associated with declining walking rates has impelled researchers from various disciplines to identify factors related to this behavior change. Currently, this body of research is in need of a transdisciplinary, multilevel theoretical model that can help explain how individual, group, regional, and physical-environmental factors all affect physical activity behaviors. To address this gap, this article offers a social-ecological model of walking that presents a dynamic, causal model of the decision-making process. Within the model, a hierarchy of walking needs operates and organizes five levels of needs hierarchically and presents them as antecedents within the walking decision-making process. This model can (a) serve as a framework by which to understand the relative significance of the cornucopia of variables identified by existing research, (b) offer hypotheses for how these factors affect peoples' decision to walk, and (c) help to guide future research and practice. (Contains 2 tables, 2 figures, and 9 notes.)