The mechanosensory mechanisms in bone include (i) the cell system that is stimulated by external mechanical loading applied to the bone; (ii) the system that transduces that mechanical loading to a communicable signal; and (iii) the systems that transmit that signal to the effector cells for the maintenance of bone homeostasis and for strain adaptation of the bone structure. The effector cells are the osteoblasts and the osteoclasts. These systems and the mechanisms that they employ have not yet been unambiguously identified. The candidate systems are reviewed here. The current theoretical and experimental evidence, which suggests that osteocytes are the principal mechanosensory cells of bone, is summarized. This evidence shows that they are activated by shear stress from fluid flowing through the osteocyte canaliculi. The evidence also suggests that the electrically coupled three-dimensional network of osteocytes and lining cells is a communications system for the control of bone homeostasis and structural strain adaptation.