In the bone marrow, hematopoietic stem cells (HSCs) are maintained by special microenvironments, termed niches. The nature and function of these niches, however, remains unclear. HSCs are thought be in contact with bone-lining osteoblasts, but recent studies have suggested that only a small subpopulation of HSCs reside in this endosteal niche. By contrast, many HSCs are associated with the sinusoidal endothelium, which is referred to as the vascular niche. Recent data have suggested that primitive mesenchymal cells, including CXC chemokine ligand 12-abundant reticular cells and nestin-expressing cells act as HSC niches. Here, we review HSC niches, with an emphasis on the emerging role of reticular niches for maintaining HSCs in a proliferative and undifferentiated state.