Cancer is dependent on so-called cancer stem cells that initiate and maintain the cancer cell population. Stem cells are described in normal tissue as low-frequent, self-renewing cells with a multi- or pluripotent differentiation potential. The true characteristics of the cancer-initiating cells are still not entirely known, but it is obvious that identifying these cells will enable us to better understand the biology of cancer. In this article, we focus on normal haematopoietic stem cells and cancer stem cells in leukaemia and multiple myeloma.