1. Pedunculate Oak trees were grown in ambient and elevated temperatures and CO2. Leaves were fed to Winter Moth caterpillars reared either in constant conditions or with the trees (caged or on-tree). 2. Caterpillars in constant conditions ate the same mass and produced the same mass of faeces whether fed elevated or ambient temperature leaves. However, less was assimilated from elevated leaves, resulting in lighter pupae and fewer, lighter eggs. 3. Caterpillars in constant conditions ate more and produced more faeces when fed elevated CO2 leaves than when fed ambient CO2 leaves, but the mass assimilated and pupal mass were unchanged. 4. Caged caterpillars reared with the trees from which they were fed had constant pupal mass in all treatments, but pupated earlier at elevated temperature. Pupal mass was also unaffected when caterpillars fed on the trees. 5. Nitrogen was reduced in both elevated temperature and elevated CO2 leaves. Increased fibre in the former prevented increased consumption and resulted in reduced pupal mass and fecundity. Reduced fibre in the latter allowed increased consumption, resulting in pupae of normal mass. 6. Despite the clear effect of nutrient quality, experiments rearing caterpillars and trees together suggest that anticipated climatic change will have no nutritional effect on Winter Moth development.