Ever since the pioneering work of Frost 21'29 it has been evident that bone remodeling in the adult skeleton is carried out by temporary anatomic structures consisting of groups of several different types of cells, referred to as basic multicellular units (BMUs). Within each BMU, osteoclasts and osteoblasts maintain the same spatial and temporal relationships between each other for many months. 43'49 The purpose of the BMU is to replace old bone, which is resorbed by osteoclasts, with new bone formed by osteoblasts, soon after and in the same location. In the uninjured adult skeleton, whether normal or osteoporotic, all osteoclasts and osteoblasts belong to a BMU and isolated unattached ones do not exist, except possibly for osteoblasts beneath the periosteurn. 2 Regrettably, many in the bone and mineral field continue to think, speak, and write about bone resorption and formation as if they were separate processes, each affecting the whole skeleton in the same way47--the independence and singularity assumptions that were demolished by Frost more than 25 years ago. 22 The organization of the BMU dictates that new osteoblasts appear only at sites recently vacated by osteoclasts. Frost sometimes referred to this phenomenon as tethering, 23 but it is now generally known as coupling. This term was coined for the close relationship between whole body rates of accretion and resorption determined by radiocalcium kinetics, 28 but is now used for the local phenomenon that is the cellular basis for the whole body observations. 42 As each BMU progresses through or across the surface of bone, the transition from resorption to formation occurs in a narrow region between the cutting and closing cones or hemicones. 43'49 During the transition, resorption is terminated by osteoclast apoptosis, 45 the surface of the resorption cavity becomes smoother and covered by a thin layer of cement substance, and mononuclear cells of uncertain nature appear. 4 These transitional events collectively comprise the so-called reversal period, manifested histologically as the reversal surface) Finally, osteohlasts assemble on the cement surface in numbers that are normally sufficient to refill the cavity. Coupling must be distinguished from balance. 3'42 The amounts of bone resorbed and formed during each remodeling cycle may become unequal for a variety of reasons, 44 a situation frequently but erroneously referred to as uncoupling rather than as focal imbalance. Successful coupling depends on two main mechanisms: one to locally increase the production of new osteoblasts, and one to ensure their arrival at a precisely defined location within a brief