This chapter establishes a general context for the rest of this book. We start with a general discussion on models and modelling process within the engineering context. We then discuss the model consistency and the modelling errors that may arise during the modelling process. Specifically, we emphasise that it is necessary to distinguish between the conceptual modelling errors, which appear because of violating basic modelling assumptions, and the instance modelling errors, which are related to the practical implementation of models. After that, we provide a brief overview of the conceptual modelling approaches, which could help us to evaluate models with respect to their coherence, as well as provide possibilities for a more formal description of the model creation process.