The fundamental basis of our work is that organs are generated by multipotent stem cells, whose properties we must understand to control tissue assembly or repair. Central nervous system (CNS) stem cells are now recognized as a well–defined population of precursors that differentiate into cells that are indisputably neurons and glial cells. Work from our group played an important role in defining stem cells of the CNS. Embryonic stem (ES) cells also differentiate to specific neuron and glial types through defined intermediates that are similar to the cellular precursors that normally occur in brain development. There is convincing evidence that the differentiated progeny of ES cells and CNS stem cells show expected functions of neurons and glia. Recent progress has been made on three fundamental developmental processes: (i) cell cycle control; (ii) the control of cell fate; and (iii) early steps in neural differentiation. In addition, our work on CNS stem cells has developed to a stage where there are clinical implications for Parkinson's and other degenerative disorders. These advances establish that stem cell biology contributes to our understanding of brain development and has great clinical promise.