In this chapter, we discuss modelling and models. What is the relation between models and systems? What is modelling? How do descriptions come into this picture? This is discussed in Sect. 3.1. Models are related to some original, which can be existing or imagined. In the same way, also the model can be mental, physical or digital. The comparison of models with their originals brings us in Sect. 3.2 to the issue of model correctness. We discuss validation, verification, the intention behind the models and their meaning. We also consider the difference between existing and imagined originals in terms of correctness.