The chemical NH3, known as ammonia, has many unusual properties which make it particularly suitable for use as a refrigerant. This paper examines these properties in the light of over 150 years of continuous use of ammonia in refrigerating systems and explains their implications for system design and operation. The constraints on ammonia use are discussed and a more rational basis for safety rules is proposed. The paper concludes with a review of other possible applications for ammonia as a refrigerant.