The impacts of insect outbreaks on tree mortality, productivity, and stand development in Canada are reviewed, emphasising spruce budworm (Choristoneura fumiferana (Clemens), Lepidoptera: Tortricidae) and mountain pine beetle (Dendroctonus ponderosae Hopkins, Coleoptera: Curculionidae). Reduced growth and survival are a function of insect population and defoliation level. It is feasible to make short-term (annual) predictions of insect population and defoliation based upon sampling, but long-term, multi-year predictions are problematic. Given the historical record, it is expected that outbreaks will occur with relatively predictable frequency and basic host relationships and abiotic constraints will not change dramatically. However, the precision of predictions at fine scales is variable and reduced over time. Relationships between tree growth reduction, survival, and cumulative defoliation or beetle population level are available for major insect species. Understanding insect outbreak effects hinges on mortality, changes in interspecies competition, regeneration, and succession. Altered stand dynamics caused by insects can be interpreted for indicators such as wildlife habitat, old forest, riparian buffer cover, viewscapes, and connectivity. Anthropogenic changes are altering impacts via range expansions, northward shifts, and changes in forest composition. We can better understand effects of insect outbreaks and how best to ameliorate damage through a combination of empirical permanent plot studies, modelling, and manipulative experiments.