In the bone remodeling process that takes place throughout the skeleton at bone multicellular units, intercellular communication processes are crucial. The osteoblast lineage has long been known to program osteoclast formation and hence resorption, but the preservation of bone mass and integrity requires tight control of remodeling. This needs local controls that ensure availability of mesenchymal precursors and the provision of local signals that promote differentiation through the osteoblast lineage. Some signals can come from growth factors released from resorbed bone matrix, and there is increasing evidence that the osteoclast lineage itself produces factors that can either enhance or inhibit osteoblast differentiation and hence bone formation. A number of such factors have been identified from predominantly in vitro experiments. The coupling of bone formation to resorption is increasingly recognized as a complex, dynamic process that results from the input of many local factors of cell and matrix origin that can either promote or inhibit bone formation