The granulocyte colony-stimulating factor receptor (G-CSFR) critically regulates granulopoiesis. Defects in G-CSFR expression due to targeted disruption of the G-CSFR gene or the genes for the transcription factors C/EBPalpha and PU.1 result in decreases in hematopoietic progenitor cell numbers and neutropenia. Mutations in the G-CSFR gene disrupt its normal signaling functions and appear to contribute to leukemogenesis. Acquired mutations in the G-CSFR resulting in truncation of the distal cytoplasmic region that mediates maturation and growth arrest signaling have been reported in patients with severe congenital neutropenia (SCN) and acute myelogenous leukemia (AML). A role for G-CSFR mutations in the pathogenesis of other disorders is speculated. This review will summarize the current state of knowledge of the G-CSFR and its role in disorders of granulopoiesis.