The circulating fluidized bed (CFB) is viewed as a system in which fine particles are transported upwards by a gas stream and then recycled to the bottom of the column. The flow structure is complex and varies widely depending mainly on the gas velocity, vessel geometry and particle size distribution. Unlike the bubbled bed, the entry, exit and wall configurations have a strong influence on the flow and mixing patterns of both phases in the column. Small particles tend to form relatively large, irregular aggregates or clusters which disintegrate and form again at appreciable frequencies. Simple core-annulus models for circulating fluidized beds assume the upflow of gas and entrained solids in a dilute central core and the downflow of dense clusters in a relatively thin annular zone near the walls. Rapid heat and mass transfer between gas and particulate solids, uniform temperature in the whole CFB, nearly a plug flow of gas and possible stepwise addition of one or more gaseous components at different levels are among the main features of the CFB reactors. Efficient combustion and pressure gasification of fossil fuels and other carbon-containing residues, meeting strict environmental requirements, are the most rapidly expanding field of CFB applications.