Hematopoiesis is the cumulative result of intricately regulated signal transduction cascades that are mediated by cytokines and their cognate receptors. Proper culmination of these diverse signaling pathways forms the basis for an orderly generation of different cell types and aberrations in these pathways is an underlying cause for diseases such as leukemias and other myeloproliferative and lymphoproliferative disorders. Over the past decade, downstream signal transduction events initiated upon cytokine/growth factor stimulation have been a major focus of basic and applied biomedical research. As a result, several key concepts have emerged allowing a better understanding of the complex signaling processes. A group of transcription factors, termed signal transducers and activators of transcription (STATs) appear to orchestrate the downstream events propagated by cytokine/growth factor interactions with their cognate receptors. Similarly, cytoplasmic Janus protein tyrosine kinases (JAKs) and Src family of kinases seem to play a critical role in diverse signal transduction pathways that govern cellular survival, proliferation, differentiation and apoptosis. Accumulating evidence suggests that STAT protein activation may be mediated by members of both JAK and Src family members following cytokine/growth factor stimulation. In addition, JAK kinases appear to be essential for the phosphorylation of the cytokine receptors which results in the creation of docking sites on the receptors for binding of SH2-containing proteins such as STATs, Src-kinases and other signaling intermediates. Cell and tissue-specificity of cytokine action appears to be determined by the nature of signal transduction pathways activated by cytokine/receptor interactions. The integration of these diverse signaling cues from active JAK kinases, members of the Src-family kinases and STAT proteins, leads to cell proliferation, cell survival and differentiation, the end-point of the cytokine/growth factor stimulus.