Available information on the breeding biology of the Great Bittern (Botaurus stellaris) is poor. New data are presented on nest description and distribution, breeding phenology, clutch and brood sizes, egg biometrics and breeding success, based on 27 nests at one French and three Italian sites. Nest shape and breeding phenology appeared at least partially linked to environmental factors. Breeding success was studied for the first time to the fledging stage using radio-tagging young. About 30% of eggs produced fledging young, and it is suggested that starvation was the main cause of chick death. Young moved away from the nest when about 15-days old and extensive dispersal occurred within 2-3 weeks from independence. A survey of the literature indicates variability in several breeding parameters and suggests that the Great Bittern is an adaptable bird.