THE RECOGNITION THAT bone size is an important deter-minant of bone strength has led inexorably to a consideration of factors that affect periosteal events. Similarly, there has been a growth in awareness of sex differences in fracture risk, and in turn, of sex-specific effects on bone size and geometry.(1) Even more recently, treatment with parathyroid hormone has been speculated to exert its antifracture effect in part through an increase in periosteal bone apposition (in addition to its major effects on trabecular bone mass), thus serving to identify the outer bone envelope as a target of therapeutic intervention.(2–4) Although imperfect, techniques have been developed that make the evaluation of bone size and geometry more feasible in clinical investigations,(5, 6) portending further interest in this area of investigation. Nevertheless, periosteal biology itself remains little investigated and poorly understood. Thus, it may be useful to consider several basic biological issues that could profoundly affect the consideration of the periosteum. The nature of the periosteum at the femoral neck is of special importance. In this discussion, the periosteum is considered a unique tissue compartment at the outer bone surface that, at least conceptually, could be the site of cellular events important for overall skeletal competence. The purpose of this perspective is to stimulate an expansion of the ways in which we consider periosteal physiology, pathophysiology, and therapeutics, specifically by re-examining common perceptions about periosteal cellular activity and their implications. Purposefully, somewhat provocative suggestions are intended to stimulate the development and testing of new hypotheses.