A common arboreal rodent of boreal and montane coniferous forests, the northern flying squirrel (Glaucomys sabrinus) has several life-history traits typical of K-selected species. Density varies among forest types, with core areas of use centering on food patches. Density is largely limited by food, and to a lesser extent, suitable natal dens, but also is influenced by potential competitors and predators. Local abundance of G. sabrinus frequently is correlated with density of large trees and snags, shrub and canopy cover, prevalence of old-forest features (e.g., coarse woody debris), and abundance of hypogeous mycorrhizal fungi (truffles). Diet varies seasonally and among habitats, but truffles (spring and autumn) and lichens (winter) are most often reported. In some parts of its geographic range, G. sabrinus has a more diverse diet and lower reliance on truffles in forests with a depauperate arboreal small mammal community. G. sabrinus is a keystone species in the Pacific Northwest, because its diet facilitates an obligate mutualistic relationship between mycorrhizal fungi and some trees and shrubs and because it is essential prey for mesocarnivores and avian predators. G. sabrinus achieves its highest densities in old growth, but also occurs in secondary forests. Disturbance that reduces structural complexity, canopy cover, or the availability of large, decadent trees typically results in smaller populations through effects on food, den sites, or risk of predation. The fundamental niche of G. sabrinus may be broader than suggested by early research in the Pacific Northwest. Sustaining viable and well-distributed populations in heavily modified landscapes will depend on the capability of remaining forest habitat to sustain breeding populations without immigration, or functional connectivity among fragmented populations such that viable metapopulations will persist. Future research should focus on identifying habitat conditions that sustain breeding populations in modified habitats and determining whether G. sabrinus can migrate freely through a matrix of unsuitable habitat.