Polymer ageing may involve physical ageing without chemical reaction occurring; chemical changes such as crosslinking during curing of a thermoset; thermal conditioning at elevated temperature; photochemical ageing, as occurs in weathering. This review concentrates on examples involving a combination of two or more of these effects, and with the consequential changes in engineering properties. Events on the molecular level lead to change in the morphology and macroscopic physical properties. For example, in a semi-crystalline polymer, chain scission caused by photo-oxidation, may lead to secondary crystallisation (‘chemi-crystallisation'), increasing density, and causing an increase in Young's modulus and reducing ductility. Similar effects may be observed in glassy polymers, due to chain scission accelerating physical ageing. Such effects are particularly common when rapid cooling during moulding has left the polymer in a state far from equilibrium. A common symptom is a change in the residual stress distribution in polymer mouldings and this is discussed. To cite this article: J.R. White, C. R. Chimie 9, 2006.