Some potential mechanisms by which bone cells sense mechanical loads are described and hypotheses concerning the functioning of these mechanisms are explored. It is well known that bone tissue adapts its structure to its mechanical load environment. Recent research has illuminated the biological response of bone to mechanical loading at the cellular level, but the precise mechanosensory system that signals bone cells to deposit or resorb tissue has not been identified. The purpose of this paper is to describe the current status of this research and to suggest some possible mechanosensory systems by which bone cells might sense environmental loads.