The best way to provide a controlled environment for museum objects is to install a full air-conditioning plant, but this frequently presents a number of problems. Often money is not available. Sometimes there are difficulties in modifying an existing building. The various aspects of the controlled environment, within the comprehensive term air-conditioning, are discussed by the author in a general order of priority headings as humidification, air-cleaning, heating, ventilation, cooling and dehumidification, full air-conditioning, costs.