Interaction of HSCs with their particular microenvironments, known as stem cell niches, is critical for maintaining the stem cell properties, including self-renewal capacity and the ability of differentiation into multiple lineages. Bone-lining osteoblastic cells (OBs) and perivascular cells have been identified as major components of HSC niches. They express cell adhesion molecules and produce paracrine factors such as Angiopoietin-1 and thrombopoietin. In this presentation, I will show the role of the endosteal niche complex for the sustaining of quiescence of HSCs, and discuss the niche regulation through reactive oxygen species (ROS).