What is a stem cell? This is becoming more than a semantic issue in biology these days, as cell therapy approaches clinical trials. It is vital to know exactly what is going on during the growth of "stem cells" in the culture dish, and how pure the population actually is. Which is the best source for generating stem cells — the blastocyst, fetus, or adult? Can they all be grown forever in the tissue culture dish — or do they change with time? Are neural stem cells generated from these tissues really the same? Clearly, the old adage "rubbish in/rubbish out" is well applied to cellular therapy. To avoid catastrophic clinical outcomes, cells must be characterized to the best of our abilities. In this presentation, the biology of "stem cells" generated from different species and different tissues will be presented. One underlying theme will be that, in many cases, stem cells are not actually being grown in culture, but instead, the more restricted "progenitor" cells—with a fixed genetic background related to their origin. Another is that human and rodent progenitor cells are dramatically different in both growth characteristics and phenotypic potential. Finally, stem cells generated from adult human tissues do not overlap seamlessly with those from embryonic sources. These differences seem to be the rule, while the rare exceptions (where cells can return to a primitive state or "transdifferentiate" between germ layers) often grab the spotlight.