The selection of the most adequate thermodynamic model in a process simulation is an issue that most process engineers have to face sooner or later. This book, conceived as a practical guide, aims at providing adequate answers by analysing the questions to be answered. The analysis (first chapter) yields three keys that are further discussed in three different chapters. The fourth chapter illustrates the phase behaviour and makes model recommendations for the most significant industrial systems. A decision tree is provided at the end of this chapter. In the last chapter, the key questions are reviewed for a number of typical processes. This book is intended for process engineers who are not specialists in thermodynamics, but are confronted with this kind of problems and need a reference book, as well as process engineering students who will find an original approach to thermodynamics, complementary to traditional lectures.