The various aspects of natural and accelerated ageing of cellulose and paper are presented and discussed. Accelerated ageing constitutes the major methodological tool for the studying of cellulose and paper ageing. The topics include: • The correlation of natural and accelerated ageing. Methodology of research, results up to date. • The chemistry of ageing: chain scission, oxidation, crosslinking. Influence of environmental factors (temperature and relative humidity), moisture content, impurities and additives, pH, light, pollution etc. • The effects on the physicochemical, mechanical, optical and structural properties of cellulose. Changes in water absorption (hornification), porosity and crystallinity. • The ageing kinetics. Chemical and property kinetics. Courses of ageing under different conditions. Autocatalysis in sealed vessels. Kinetic models. The applicability of the Arrhenius studies for paper permanence predictions and the related controversy are discussed. • The methods of accelerated ageing. Standards. Dry and humid ageing, sealed enclosures. • Applications of accelerated ageing: permanence predictions, classification of papers according to their permanence, performance standards for testing for paper permanence, evaluation of paper conservation interventions.