Fluidization regimes were studied in liquid-solid and gas-Liquid-solid fluidized beds. The liquid velocities at which regime transitions occur in liquid-solid and gas-liquid-solid systems were obtained for monosize and multicomponent systems. Minimum fluidization, complete fluidization and complete mixing velocities of particles were obtained from pressure drop measurements, a collision technique or a conductivity method. The collision technique provided accurate complete fluidization and complete mixing velocities by measuring directly the particle motion. The conductivity technique, which is easier to implement, gave similar but less accurate results.