The three ways in which temperature and relative humidity are measured in National Trust houses are described. The most recent of these are battery-operated electronic data loggers. They have vastly increased the amount of information on conditions, and have facilitated improvements to the environment in many of the 200 houses. Three case studies illustrating methods of direct control of relative humidity using dehumidifiers are described. The first involves a storage area in a house undergoing restoration, in which a Polythene tent was constructed, and the relative humidity controlled within it, using a refrigerative dehumidifier. The second describes the controlling of the environment inside a Perspex display case containing a green baize and silk embroidery textile, by circulating dry air from a reservoir produced by a dehumidifier. The final case study shows the importance of draught-proofing for the successful use of dehumidification in a building. Finally, the possibilities for controlling relative humidity by temperature are discussed, and found to be particularly suitable for most rooms in historic houses. -- AATA