Gas-solids circulating fluidized beds have been successfully used in catalytic cracking of heavy oil, coal combustion, and some metallurgical and physical processes (Grace, 1990). Gas-liquid-solids fluidized beds are operated mainly in conventional fluidization regimes without solids circulation or in the transport regime with low solids holdups (less than 5%) (Fan, 1989). A circulating/fast fluidization regime, however, has not been studied. A three-phase circulating fluidized bed has several potential applications in biochemical and chemical processes. Three-phase fluidized-bed bioreactors generally use light and small particles (Berk et al., 1984). Circulating operation can promote solids mixing and increase product throughput per unit bed cross section, while high shear stress can promote biofilm renewal (Pirozzi et al., 1990). In three-phase hydrotreating reactors, solids catalysts lose their activity due to the deposit of metal and coke on the surface. Circulating operation not only regenerates deactivated catalyst continuously using accompanying downcomers but also transfers heat to and from the reactor. This article discusses the flow regimes of the three-phase circulating fluidized bed.