Hematopoietic growth factors are the major regulatory molecules supporting constitutive and inducible hematopoiesis. Molecular biology techniques have led to the identification and cloning of a series of hematopoietic growth factor genes, and the synthesis of large amounts of these molecules has facilitated investigations or their physiological functions and the network of their interactions. The present article focuses on the role of hematopoietic growth factors, growth-factor-synergizing factors, and growth-factor-releasing factors in the regulation of proliferation, differentiation, and functional activation of hematopoietic stem cells and lineage-committed progenitor cells. Cellular sources of growth factors, signal transduction pathways involved in their gene activation and transcriptional and posttranscriptional mechanisms that govern regulation of expression of these genes are also detailed.