We review the molecular mechanisms involved in response to ionizing radiation in various hematopoietic cell types. First, a general overview of the radiation induced cell signaling molecules in mammals is given. The importance of highly conserved kinases, such as ATM and ATR, as well as the p53 protein for maintaining the genome stability is highlighted. Next, particular attention is given to radiation-induced gene expression in the hematopoietic system. For some hemopoietic cell subpopulations, recent data are provided which might explain the differential radiosensitivity of these cells. Finally, radiation-induced cytokines are reviewed, as they affect the global response to radiation.