Steady‐state hematopoiesis responds to extracellular stimuli to meet changing demands and also to pathologically altered intracellular signaling. Granulocyte production increases following infection or in response to cytokine stimulation, and activation of the CCAAT/enhancer‐binding protein β (C/EBPβ) transcription factor is required for such stress‐induced granulopoiesis, whereas C/EBPα plays a critical role in maintaining steady‐state granulopoiesis. Different roles of these C/EBP transcription factors in different modes of hematopoiesis are evolutionally conserved from zebrafish to humans. In addition to reactions against infections, C/EBPβ is responsible for cancer‐driven myelopoiesis, which promotes cancer progression, at least in part, by abrogating the immune response in the cancer microenvironment. The BCR–ABL fusion protein activates emergency‐specific pathway of granulopoiesis by upregulating C/EBPβ. This in turn causes chronic phase chronic myeloid leukemia, which is characterized by myeloid expansion. The C/EBPβ transcription factor also plays a role in other hematological malignancies of both myeloid and lymphoid lineage origin. Thus, elucidation of the upstream and downstream networks surrounding C/EBPβ will lead to the development of novel therapeutic strategies for diseases mediated by non‐steady‐state hematopoiesis.