Let there be light. Pluripotent and determined stem cells as well as progenitor cells look morphologically all alike. However, if we cytometrists turn on the light and use the right characteristics and their combination, we can tell what the future of these progenitor cells will be. This focus issue of Cytometry Part A unravels the secrets of some of the most relevant types of these cells, and how to distinguish and isolate them. See the articles by the experts in their fields, in this issue.