Stem cells have generated more public and professional interest than almost any other topic in biology. One reason stem cells capture the imagination of so many is the hope that understanding their unique properties may reveal paths leading to new treatments for a variety of degenerative illnesses. And although the field of stem cell biology has grown rapidly, considerable confusion and disagreement as to the nature of stem cells exists. This confusion can be partly attributed to the sometimes idiosyncratic terms and definitions used to describe stem cells. Although definitions can be restrictive, they are useful when they provide a foundation for mutual understanding and experimental standardization. This chapter presents some definitions of stem cells, discusses the origin of stem cells, and suggests criteria or standards for identifying, isolating, and characterizing stem cells. Finally, the notion of ‘stemness’ and its possible application in understanding stem cell biology is explained.