Hematopoiesis is a complex, highly regulated process in which a small number of primitive stem cells produce all the mature blood and immune cells required by an animal throughout its life. In this review, we summarize current understanding of human CD34+ CD38- cells, a small subclass of hematopoietic cells, which is enriched for primitive precursors, including stem cells. This review emphasizes functional, molecular and immunophenotypic characteristics of the cells and includes some of the factors which are believed to regulate the survival, growth and differentiation of this important class of cells.