Thousands of stress measurements on canvas paintings have been collected over the past three years using an automated logging program. They confirmed the observation that paint needs a stiff substrate in order to be preserved undamaged. Considerable fluctuations in the tension of stretched fabrics in response to changes in temperature and RH were recorded. Such fluctuations lead to repeated overstretching of the canvas during stress peaks. With each overstretching, the canvas loses some of its initial tension. With time, the canvas becomes so slack as to be incapable of providing a firm support for the paint film. No longer restrained in its expansions and contractions, the paint film distorts, and pulls the canvas to which it is attached. Stress diagrams of canvas samples following stretching, sizing, priming and aging show distinctly different patterns which support the above findings. Accordingly, in order to maintain the canvas as an effective support for the paint film, preventive conservation must preserve its stiffness. This can be achieved by the following means used either separately or in combination: (1) reducing stress fluctuations by strictly controlled air-conditioning; (2) providing the paint film with a stiffer support, as by lining; (3) eliminating stress peaks and maintaining sufficient tension with springs or other devices.