The fundamentals concerning the influence of oil on the various components of a refrigerant plant will be discussed generally; especially the favorable and unfavorable influence of the lubricant will be dealt with. Further, the requirements for the lubricating oils will be described, as they can be deduced from the fundamental discussion. Additional requirements for the oil will be dealt with concerning new developments; for example, large heat pumps, high temperature heat pumps, and plants with refrigerant mixtures as working medium for cooling systems as well as for heat pumps. For these new systems, our investigations concerning lubricants with refrigerant mixtures will be presented.