The role of growth factors in controlling the development of glial cells in both the peripheral and central nervous systems has been investigated for a number of years. The recent discovery of a new family of growth factors termed the neuregulins (NRGs) has led to an explosion of information concerning the putative role of these growth factors in the development of Schwann cells (SC), oligodendrocytes (OLG), and astrocytes. Many of these previous studies have focused on the effects of exogenous NRGs on glial cell development and differentiation. We now review the evidence that these glial cells themselves produce NRGs and discuss the major implications of these findings with respect to glial cell development and diseases which affect glial cell function. We also discuss the potential role of endogenous NRGs following neural injury.