This paper examines the essentials of the biology, impact and control of the European tree-killing wood wasp Sirex noctilio, which was found established in exotic Pinus radiata plantations in Australia about 1950-51. The wasp, with its pathogenic symbiont Amylostereum areolatum, has severely damaged some unthinned pine stands of intermediate-age, and valuable shelterbelts on farmland in Tasmania and Victoria. One-year life cycles are predominant, but three-month cycles and two or three-year cycles are also known. Emergence of adults, followed by attack on pine, occurs mostly between mid and late summer, when soil moisture levels, growth rates and tolerance of pine to pests and diseases are low. Various volatile substances produced by phloem/cambium tissues of stems and large branches, are important in the attraction of the pest. Susceptible trees are normally physiologically stressed, and attack on them is associated with a reinforcement of stress due to mucus injection, and a subsequent insect-pathogen development phase in the wood.In Tasmanian plantations, the introduced ichneumonid, ibaliid and stephanid parasitoids, and the parasitic nematode Deladenus siricidicola, have been effective control agents, though some outbreaks have collapsed even without natural enemies. In some parts of Victoria, where droughts are frequent and summer temperatures are high, an ecological balance between host and parasite populations may occur only after an unacceptably high level of tree mortality in stands that have remained unthinned due to unfavourable markets and unavailability of funds for sanitation felling. For such stands, a control strategy is being devised that will combine the effects of established biological control agents and minimum, selective noncommercial thinning, with the strategic placement of groups of trap trees, which have been predisposed to S. noctilio attack by injection with herbicide during spring.