This article discusses the important characteristics of fluidized beds. The total space occupied by a fluidized bed can be divided into three zones: grid zone, main zone, and above-bed zone. The article discusses the various types of atmospheres of fluidized beds, such as oxidizing and decarburizing atmosphere; nitrocarburizing and nitriding atmosphere; carburizing and carbonitriding atmosphere; and chemical vapor deposition atmosphere. External resistance heating, external combustion heating, internal resistance heating, direct resistance heating, submerged combustion heating, and internal combustion heating can be used to achieve the heat input for a fluidized bed. The article also describes the operations, design considerations, and applications of fluidized-bed furnaces in heat treating. Thermochemical surface treatments, such as carburizing, carbonitriding, nitriding, and nitrocarburizing, are also discussed. Finally, the article reviews the principles and applications of fluidized-bed heat treatment.