Trembling aspen (Populus tremuloides Michx.) is the most important deciduous tree in the Canadian boreal forest, with >1000 Tg of carbon stored in the aboveground biomass of this species. Since the early 1990s, aspen dieback has been noted over parts of the southern boreal forest and aspen parkland in western Canada. In this study, tree-ring analysis and forest health assessments were conducted in 18 aspen stands near Grande Prairie, Alta., to examine causes of reduced growth and dieback. Defoliation histories were reconstructed based on light-colored ("white") tree rings and records of past insect outbreaks. The results indicated that several factors contributed to the observed dieback. Defoliation by forest tent caterpillar (Malacosoma disstria Hbn.) and drought in the 1960s and 1980s led to reduced growth and predisposed some stands to secondary damage by wood-boring insects and fungal pathogens. Thawfreeze events during a period (19841993) of unusually light snow cover in late winter may have also contributed to the observed dieback. Under global change, the severity of these stressors may increase, which would pose a serious concern for the future health, productivity, and carbon sequestration of aspen forests in the region.