The paper presents a critical review of various current concepts of the structure and kinetics of unmyelinated nerve fiber. A classification of nerve fibers, different from the earlier ones, is proposed, that demonstrates not only the morphological fiber types, but also the kinetics of their reversible transition stages from non-glial to myelinated fiber. Evidence is presented to show the erroneousness of conceptions, still appearing in many publications, that consider the unmyelinated nerve fiber as "the Remak's cable type fiber". According to the current data, "Remak's fiber" is a glial-neurite complex, i.e. a bundle of unmyelinated nerve fibers covered with a single glial cell. Using the electron microscope, it was demonstrated that comparable glial-neurite complexes of myelinated nerve fibers, formed in CNS in a similar way by a single oligodendrocyte, cannot be named a single fiber. Cutting the nerves makes visible that the single fibers forming "the Remak's fiber" stem from different cells, therefore they cannot be a single "fiber". It has been shown for the first time experimentally, that in extreme situations, as a result of contraction of gliocyte processes, unmyelinated fibers may "leave" the glial-neurite complexes and become the nonglial fibers. Some data are presented that may serve as criteria for differentiation of unmyelinated fiber with a stratified sheath from developing myelinated fiber.