jats:p This article reviews the physical principles of stem cell populations as active many-particle systems that are able to self-renew, control their density, and recover from depletion. We illustrate the dynamical and statistical hallmarks of homeostatic mechanisms, from stem cell density fluctuations and transient large-scale oscillation dynamics during recovery to the scaling behavior of clonal dynamics and front-like boundary propagation during regeneration. </jats:p>