Impact of forest pests and fire on stand growth and timber yield is reviewed, with emphasis on spruce budworm (Choristoneurafumiferana(Clem.)). Damaging agents reduce tree growth, kill trees, destroy the commercial value of stands, and sometimes reduce yield in subsequent rotations. Sustainable harvest may be reduced by up to 60% by a severe spruce budworm outbreak and up to 40% by a 1% per annum loss to fire. Serious overestimation of future timber supply can therefore result from the failure to allow for catastrophic or continual small losses caused by fire or biotic agents. Current efforts in New Brunswick to explicitly incorporate effects of spruce budworm defoliation into forest management planning are described, including research studies into protection planning and delivery, damage detection, and defoliation-based growth forecasting. An improved understanding of the impact of insects, disease, and fire on stand yield and methods to incorporate this information into timber supply analyses are essential to reduce uncertainty about future timber supply.