The bone healing process normally unites fractures, arthrodeses, osteotomies, and bone grafting operations. The process normally proceeds in successive stages named the fracture, granulation, and modeling/remodeling stages. A separate regional acceleratory phenomenon speeds up each of the other stages. The osteoclast and osteoblast cells that make intercellular substances of each stage do not exist in sufficient numbers to heal the bone at the moment of fracture or operation. They are made by local multicellular mediator mechanisms that contain precursor and supporting cells, capillaries, lymph, and innervation, plus local autocrine and paracrine regulation. Under the influences of local and systemic agents, these mediator mechanisms determine whether new local osteoclasts and osteoblasts will appear, in addition to when, where, how many, what kind, and for how long. Errors in those functions can then lead to several kinds of retarded or otherwise abnormal bone healing that will be discussed in Part II of this work.