Spruce budworm (Choristoneura fumiferana) is a native insect that defoliates needleleaf trees, especially balsam fir (Abies balsamea) and spruces (Picea spp.), in northern North America. Spruce budworm can defoliate millions of hectares of forest during an infestation, depressing regional economies that depend on the timber industry. Ecosystems, though, can benefit from spruce budworm because outbreaks rejuvenate the forest, maintaining optimal levels of primary production, and thereby carbon sequestration. Although many ecologists, entomologists, geographers, and resource managers have studied the effects of spruce budworm on spruce–fir forests throughout the region, no single explanation of what causes the number of insects in a forest to rise and fall is universally accepted. Spruce budworm populations can reach ‘outbreak’ levels, or densities high enough to defoliate and kill balsam fir and spruce on a landscape scale, on average every 30–40 years. We review the biology of spruce budworm, the processes that scientists follow to reconstruct spruce budworm outbreaks, the leading hypothesis to explain population dynamics and outbreak events, and the complexity of forecasting possible future trends of populations and distributions of spruce budworm. Reconstructions of past outbreaks help us understand their severity, frequency, and spatial synchrony, which might be linked in complex ways to climate, forest, and stand characteristics. Future spruce budworm dynamics are difficult to predict because the insect is part of a complex food web. In the coming decades, spruce budworm probably will survive climate change because it is adapted to a wide range of temperatures and precipitation amounts.