Animals and plants maintain small pools of stem cells that continuously provide the precursors of more-specialized cells to sustain growth or to replace tissues. A comparison of plant and animal stem cells can highlight core aspects of stem-cell biology. In both types of organism, stem cells are maintained by intercellular signals that are available only in defined regions (niches) in the tissues. Although plants use different signals and are more flexible at establishing stem-cell niches in new locations, recent evidence suggests that the mechanisms restricting cell fate in stem-cell progeny are similar in both kingdoms and might pre-date the evolution of multicellular organisms.