The reproductive biology of the critically endangered helmeted honeyeater was documented in and near the Yellingbo State Nature Reserve, Victoria, from 1984 to 1993. The population bred in pairs, sometimes with helpers. Females did most of the nest construction, incubation and brooding; both parents fed the young and males more often defended the nest. Nests were cup-shaped and placed in shrub thickets, or less commonly in reedbeds, ferns or eucalypt foliage. In all, 91% of clutches were of two eggs. Young fledged from 33% of nests, estimated by the Mayfield method. Predation was the main cause of nest failure, with adverse weather also a significant contributor. Post-fledging survival was high. Juveniles were substantially independent by the sixth week after hatching. The helmeted honeyeater was markedly multi-brooded, with re-nesting usually occurring rapidly after both failure and success. The commitment by individual pairs of helmeted honeyeaters to reproduction can extend to a predictable 70% of the year. This level of commitment is probably facilitated by their sedentary, territorial nature and the moisture-stable environment occupied. Reproductive performance does not limit the helmeted honeyeater population.