Bone resorption of residual ridges is common. The rate of resorption varies among different individuals and within the same individual at different times. Factors related to the rate of resorption are divided into anatomic, metabolic, functional, and prosthetic factors. Anatomic factors include the size, shape, and density of ridges, the thickness and character of the mucosal covering, the ridge relationships, and the number and depth of sockets. Metabolic factors include all of the multiple nutritional, hormonal, and other metabolic factors which influence the relative cellular activity of the bone-forming cells (osteoblasts) and the bone-resorbing cells (osteoclasts). Age, sex, and general health are inadequate to describe the bone factor but do give some clinical clues. Functional factors include the frequency, intensity, duration, and direction of forces applied to bone which are translated into cellular activity, resulting in either bone formation or bone resorption, depending on the patient's individual resistance to these forces. Prosthetic factors include the myriad of techniques, materials, concepts, principles, and practices which are incorporated into the prostheses. Although the various factors can be divided into these four groups for academic purposes, they are all interrelated, and any one factor may be evaluated only if we place it in its proper perspective to all factors. Since bone resorption depends on the response of living cells to force, the more basic sciences concerned with the physiology and pathology of cells are understood, the more educated will be our clinical judgment.