The understanding of complex stellar populations and their star formation histories relies on the stellar evolution theory as much as the improvement of our knowledge of the stellar evolution relies on the study of stellar populations. In this context, testing stellar evolution models is fundamental to properly evaluate the reliability of the results of the analysis of observational data and also as a way to improve the models themselves. Here we present a few examples among the several ones that could have been selected showing strengths and weakness of the stellar evolution models. They include direct comparison of predictions of different libraries; relevant properties of stellar populations which interpretation depends on not well established results of stellar evolution, and comparison of solutions of the star formation history of complex stellar populations using different sets of models. We have adopted the observer point of view, but our review is intended both for observers and theoreticians with the aim of stimulating the former to provide stronger observational constraints where they are needed and to encourage the latter to isolate the input physics responsible for the different behavior between models and the reasons for the discrepancies with data.