:  The bone–immune interface has become a subject of intense interest in recent years. It has long been recognized that infection, inflammation, and autoimmune disorders are associated with systemic and local bone loss. Yet, it is only recently that T lymphocytes and their products have been recognized as key regulators of osteoclast formation, life span, and activity. Similarly, sex steroids and aging have been known to regulate the immune system and T cells for decades. In spite of the abundance of clinical and physiological clues, it is only in the last few years that investigators have linked immune cells to the etiology of postmenopausal and senile osteoporosis, as well as to the bone loss caused by a variety of endocrine conditions. As surprising is new evidence showing that in contrast to their bone destructive effects under certain pathological conditions, T cells are highly protective of basal bone homeostasis, through complex regulatory effects on osteoprotegerin (OPG) production by B cells, involving CD40 to CD40 Ligand (CD40L) costimulation. This article examines the experimental evidence suggesting that estrogen prevents bone loss by regulating T cell function, and that T cell costimulation with B cells is critical for OPG production and maintenance of basal bone homeostasis.