Fluidization represents an important particulate and multiphase operation, featuring dynamic interactions between a continuum fluid and a discrete phase. It is typically realized in a vertical column or pipe. Various fluidization regimes occur, depending on the property of the fluidizing particles, flow rate, and external field force applied. This chapter describes gas–solid fluidization represented by dense-phase fluidized beds and circulating fluidized beds. Fluidization under the gas–liquid–solid flow conditions is also illustrated with the inclusion of its limiting condition of two-phase flows. Basic topics of fluidization include the fluidization regime classification and characteristics, phase-interaction mechanisms in the dense and dilute phase fluidization as well as nanoparticle fluidization, fluidized bed systems, and multiscaled transport phenomena, such as clustering, agglomeration, breakup, and coalescence of dispersed particles or bubbles. For the numerical modeling of fluidization systems, the Eulerian–Eulerian modeling is extensively used and often coupled with the DEM models or kinetic theory models for collision-induced transport in the dispersed phase.