The rotation rate of a cool star provides an additional method beyond the usual ones to characterize and understand its behavior. Increasingly large samples of rotation periods are now emerging from both ground- and space-based work. This talk will present some systematic behaviors of these rotation periods, including dependencies on age and mass, especially as probed by open cluster studies. The results suggest that it is often possible to derive an age for a cool star (and its planets) from such rotation period measurements, via a procedure called gyrochronology. This talk addresses the possibilities of this emerging area of work, and also certain complexities and cautions that should be considered in this Kepler, TESS, and soon, PLATO era.