The selection of the most adequate thermodynamic model in a process simulation is an issue that most process engineer has to face[sic] sooner or later. This book, conceived as a practical guide, aims at providing adequate answers by analysing the questions to be looked at. The analysis yields three keys that are further discussed in three different chapters. A good understanding of the properties required in the process, and thier method of calculation is the first key. The second chapter provides to that end in a synthetic manner the most important equations that are derived from the fundamental principles of thermodynamics. An adequate description of the mixture, which is a combination of models and parameters, is the second key. Finally, a correct view of the phase behaviour and trends in regard of the process conditions is the third key.