This article provides a detailed description of the glial cell types in the nerve fiber layer of the main olfactory bulb during embryonic development, in adult mammals, and at the nerve entry zone of the first cranial nerve. In adult mammals, the glial cell types of the olfactory nerve fiber layer include intrafascicular ensheathing cells, which have the exclusive role of ensheathing the olfactory axons in both the PNS and CNS, and interfascicular astrocytes, which occupy the spaces between adjacent olfactory fascicles. The ensheathing cells are particularly interesting because they possess a mixture of Schwann cell and astrocytic phenotypic features, are more likely to be of placodal than of CNS origin, and have the exclusive role of forming the glia limitans at the PNS-CNS transitional zone. It is proposed that one important function of ensheathing cells is to modulate the growth of olfactory axons within the CNS; this modulation is probably mediated by selective cell adhesion molecules, extracellular matrix molecules, and chemotropic agents.