This paper tests the predictions of parental investment theory and other hypotheses relating to variation in brood defence by examining aggression displayed by great skuas Catharacta skua towards intruders within their territories. Aggression serves the function of nest defence; hatching success of adults breeding in Shetland increased with aggression displayed during incubation, and the correlation between aggression and hatching success was apparent in three separate age classes. Adults displayed higher levels of aggression and greater parental investment in reproduction in years of poor food supply. This was not due to an increase in adults' expectations of future benefits of brood defence with increased investment. since hatching success was unaffected by food supply, and breeding success was lower in years of poor food supply. The observed increase in aggression therefore supports parental investment theory. Aggression increased with body condition index (in terms of mass corrected for body size) for females, but decreased with increasing body condition index for males. This probably reflects size-specific differences in the relative benefits of weight and manoeuvrability as means of promoting effective brood defence and reducing the risk of injury to parents. Aggression may also reflect adult quality, with body condition reflecting quality in opposite ways in males and females, as a result of their different roles during the breeding season.