Pressurized fluidized bed combustion (PFBC) is an emerging energy conversion technology for solid fuels, well suited to low-grade fuels and waste materials. The chemical and thermal behaviour of fluidized bed combustors is strongly influenced by the hydrodynamics. The fundamental hydrodynamics of fluidization are the subject of this article. Most PFBC units operate in the bubbling regime, so we concentrate on bubbling fluidization, but also include information on behaviour outside current operating practice. In addition to fundamental hydrodynamics, mixing, heat and mass transfer, elutriation and entrainment, and reactor models are treated.