Leukemia-initiating cells (LICs) or leukemia stem cells (LSCs) are defined by their ability to form tumors after xenotransplantation in immunodeficient mice and appear to be rare in most human leukemias. In various leukemias, only small subpopulations of cells can transfer disease upon transplantation into immunocompromised NOD/SCID mice, and markers that distinguish the leukemogenic cancer cells from the bulk populations of non-leukemogenic cells have been identified. However, the phenotype of LICs is heterogeneous: it is variable for the different types of acute myeloid leukemias; cells with different membrane phenotype can act as LICs in each B-acute lymphoid leukemia; LICs change during the evolution of chronic myeloid leukemia from the chronic to the acute phase. There is a general consensus that the identification and characterization of leukemic stem cells might lead to the identification of new therapeutic targets and, through this way, to more effective treatments by focusing therapy on the most malignant cells.