This review paper describes a brief historic perspective of transition from natural refrigerants to synthetic and now back to natural refrigerants. The thermodynamic, physical and safety properties of ammonia are presented. The advantages and disadvantages of ammonia refrigerant are discussed. The heat exchanger technology has seen introduction of compact and efficient equipment both in shell and tube and plate technologies. The micro-channel exchangers are being introduced in the automotive industry and it could soon enter into the commercial and industrial refrigeration markets. Compact heat exchangers are of interest due to their low refrigerant inventory features. A new electromotor technology is being developed now, using encapsulated copper windings or aluminum windings for semi-hermetic ammonia compressors. A hermetic scroll compressor for ammonia small refrigerating systems is also in the offing. In the last several years large numbers of air conditioning systems with ammonia chillers have been installed in commercial and public facilities. In large industrial systems where there is a need for low evaporating temperatures, Ammonia/Carbon Dioxide cascade systems are being proposed and installed.