Intercellular communication is critical to the development of bone cells and the regulation of bone cell function to maintain a normal bone mass. Several specific mechanisms and molecules involved in communication among osteoblasts and between osteoblasts and osteoclasts have been recognized. Osteoblasts and osteoclasts may interact by mediators released by one cell and act on another, by membrane molecules on both cells interacting directly, and by gap junctional communication. Substantial evidence shows that bone cells use all three of these mechanisms. This article discusses recent advances in research regarding communication between bone cells by extracellular nucleotides/purinergic P2 receptors; gap junctional communication; and the relationship to bone cell function, signaling by intercellular calcium waves, and direct cell-cell interactions by cell- adhesion molecules. Also, the recently identified osteoprotegerin ligand- osteoclast differentiation factor important to osteoclast differentiation is discussed.