Stem cells have been identified as a source of virtually all highly differentiated cells that are replenished during the lifetime of an animal. The critical balance between stem and differentiated cell populations is crucial for the long-term maintenance of functional tissue types. Stem cells maintain this balance by choosing one of several alternate fates: self-renewal, commitment to differentiate, and senescence or cell death. These characteristics comprise the core criteria by which these cells are usually defined. The self-renewal property is important, as it allows for extended production of the corresponding differentiated cells throughout the life span of the animal. A microenvironment that is supportive of stem cells is commonly referred to as a stem cell niche. In this review, we first present some general concepts regarding stem cells and their niches, comparing stem cells of many different kinds from diverse organisms, and in the second part, we compare specific aspects of hematopoiesis and the niches that support hematopoiesis in Drosophila, zebrafish and mouse.