In this block we introduce the Buckingham theorem, which is essential to derive as simple as possible models and models having sense. Unfortunately, this theorem is not known by a large number of people involved in modeling problems, when this theorem should be included in the high school material, due to its importance. A detailed description of the steps to be followed to derive a model is included, incorporating an explanation of how to obtain dimensionless ratios and how to make dimensional analysis tables, which are important in the application of the theorem to modelling. The theorem is illustrated with several examples, including the Sonin Ball example, the simply supported beam, the flexible retaining wall, the crack grow model and some counter-examples. For the latter, the problem and resulting inconveniences of using these models are discussed.