Recent advances in cell isolation and culture procedures, combined with growing understanding and use of molecular biology and biochemistry techniques, have resulted in the establishment of a new field of biological/biomedical research: cellular and tissue engineering. In the biomaterials field, cell and tissue bioengineers are investigating the development of proactive biomaterials (for example, bioceramics, chemically modified implant metals, and biodegradable tissue scaffolds) which utilize cellularor molecular-level methods of manipulating cell/tissue behavior in order to encourage clinically desirable biological events at the tissue-implant interface. In vitro investigations utilizing osteoblasts, osteoclasts, and appropriate precursor cells, combined with long-term (i.e., years) tis- sue engineering studies in vivo are needed t o enhance current understanding of the many mechanisms involved in bone formation and regulation. Such understanding will allow the development of proactive biomaterials for use in bone, which can elicit specific, timely, and clinically desirable responses from surrounding cells and tissues.