Most of what we know about the stellar population of nearby, resolved galaxies comes from the interpretation of their color-magnitude diagrams, by comparison with stellar evolutionary models. We review how well current stellar evolution models reproduce the properties of simple stellar populations. Emphasis is given to the regions of the color-magnitude diagram which are most useful for deriving age, metallicity, or distance of a population. Extensive comparison is made between the predictions of the most-used stellar evolution libraries, in order to estimate how model dependent the results are. The present review, written from a user perspective, aims at emphasizing the strengths and weaknesses of the models, and is intended both for observers and theoreticians. We hope to encourage observers to provide stronger observational constraints where they are needed, and to stimulate theoreticians to isolate the input physics responsible for the different behavior between models and the reasons for the discrepancies with data.