Hematopoietic stem cells were once considered identical. However, in the mid-1990s, it became apparent that stem cells from a person's early developmental phases are superior to those from adults, and aged stem cells are defective compared with young stem cells. It has since become clear that polycomb group proteins are important regulators of stem cell functioning. Polycomb group proteins are chromatin-associated proteins involved in writing or reading epigenetic histone modifications. Polycomb group proteins are involved in normal blood cell formation, in cancer, and possibly in aging. In this review, we describe how the different phases of hematopoietic stem cells-birth, maintenance, functional decline, derailment, and death-are continuous processes that may be controlled by polycomb group proteins.