Cellulose has the environmental advantage that it is bio-degradable and that paper made from it is readily recycled by repulping. It is perhaps surprising that cellulose keeps as well as it does and that some books keep for centuries. In spite of much research work, it is impossible to predict the permanence of individual samples really precisely from artificial ageing tests, though the factors to affect ageing are fairly clear. The quality of the furnish has always been known to be important. Acidity has been studied for a long time and the permanence of existing books can be improved, at a cost, by neutralisation. Storage temperature is important, but has been largely ignored, although it could be corrected in many existing libraries.