A diverse spectrum of complementary experimental investigations has demonstrated that two major cytokine superfamilies, the transforming growth factor-beta (TGF beta) and the hemopoietins, mediate an extensive range of developmental events in the nervous system that often rivals and frequently exceeds that of the classic neurotrophins. The exponential growth in the identification and physiological analysis of TGF beta subclasses of cytokines that transduce intracellular signals through intrinsic membrane serine/threonine kinase-associated receptor subunits has led to an increased understanding of their complex cellular actions in programming the temporospatial expression and maturation of anatomically distinct neuronal subpopulations. An analysis of the developmental parallels that exist between neural development and hematolymphopoiesis has fostered an expansion in the identification and classification of cellular actions of hematolymphopoietic cytokines that are also active during neural development. During the process of neural maturation, it has also become apparent that an extensive range of cell surface-associated and intracellular signalling molecules that are essential for hematopoietic and immunological development may also represent an important set of effector molecules that are active during neuronal differentiation. These recent advances in cell and molecular biology have allowed us to begin to construct an integrated model of the developmental signalling pathways and diverse cellular processes that are necessary for graded stages of neuronal maturation. These cumulative observations suggest that a dynamic hierarchy of epigenetic and genetic signals is essential for the growth, survival, and maturation of regional neuronal subpopulations that are derived from multipotent progenitor species within the central and peripheral nervous systems.