Studies related to the treatment of pressure sores have appeared in the literature. Questions remain regarding both the etiology and treatment. This article presents a conceptual schema within which current knowledge can be organized and examined and further study facilitated. The critical determinants of pressure sores are primarily the intensity and duration of pressure. A second critical concept, the tolerance of the tissue for pressure, is also discussed. Conditions contributing to prolonged and intense pressure are included within the concepts of mobility, activity, and sensory perception. Tissue tolerance for pressure is influenced by both extrinsic and intrinsic factors. Extrinsic factors such as moisture, friction, and shear impinge on the skin and underlying tissues, while the multiple influences of nutrition, the physiologic effects of stress, aging, and alterations in cellular respiration are intrinsic factors which influence the architecture and integrity of the skin and supporting structures. The schema provides broad categories capable of organizing existing knowledge and new findings.