In this book CFD simulations of the hydrodynamic behaviour of a fluidized bed reactors are described and shown. Two main topics have been analyzed: the transition from bubbling to slugging fluidization regimes (in the first section) and the evolution of volatiles from a submerged gas-emitting particle (in the second section). Both the analysis have been performed with a commercial code (CFX 4.4) using the granular kinetic theory. The fundamental principles of fluidization and the two-phase flows physics as well the basic approaches to computational fluid dynamics modelling and numerical methods of differential equations solution have been presented in the firs introductory chapters. In the following sections the computational results about the bubbling to slugging transition have been presented and compared with theoretical predictions and a good agreement has been noticed. Moreover the hydrodynamic behaviour of a submerged gas-emitting particle has been studied in a qualitative manner. Finally also the computational files used in this study have been reported.