A model is a simplified mathematical representation of a phenomenon that simulates its functioning. Science has long used, explicitly or implicitly, approaches that associate models with pure theory and experimentation. These three lines of research are inseparable. We expect a theory to explain the experimental results and predict new results; from an experiment we generally expect it to verify the validity of existing theories and to collect new data. With the exception of a few cases where experimentation is not possible, the confrontation of a model with experimental data is needed to validate it. Sometimes, however, this comparison is misleading.