Some observations and experiments concerning the adaptation of living bone tissue to its load environment are summarized. Two compatible continuum models for bone adaptation processes are described. The first model, the theory of surface remodeling, is intended to model the processes by which the applied loads induce shape changes in the whole bone. The second model, the theory of internal remodeling, is intended to model the processes by which the applied loads induce changes in the bulk density of the bone tissue these theories are presented.