Agents, not Time, act on paintings. The overt ones have always been understood, the subtle ones only recently: light; humidity; warmth; and pollution. If they are to be controlled, there must be a perceived need and a will to act. Attitudes in galleries have varied between two extremes: an expedient fatalism that gives precedence to the immediate public, and an obsessive caution that gives precedence to an unspecified future. Polemics develop partly because rate of deterioration has been vague. This article reviews the current state of knowledge, and suggests guidelines for preventive conservation. Some are familiar: ideal relative humidity (RH) is 50%, ultraviolet (UV) filters are essential, backing boards and glass give the best control of RH and pollution. Some are unfamiliar: 10% RH fluctuation should cause little cracking per century, a contemporary artist may use a palette that does not fade for centuries at several times conservation levels, figurative paintings of the last few centuries are often fading, just perceptibly, every few decades even at conservation light levels.