Stellar Population Models

This review deals with stellar population models computed by means of the evolutionary synthesis technique that was pioneered by Beatrice Tinsley roughly three decades ago. The focus is on the simplest models, the so called Simple Stellar Populations, that describe instantaneous generations of single stars with the same chemical composition and age. The development of these models until very recent results is discussed, pinpointing the model uncertainties that have been solved and those that still demand a cautionary use of the models. The fundamental step of calibrating the models with galactic globular clusters, for which ages and element abundances are known independently, is illustrated by means of key examples.