  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.
