Sudden dieback and deterioration of mature aspen stands is commonly observed throughout North America. This dieback process has tremendous ecological and economic importance, yet remains poorly understood. This paper summarizes our understanding of aspen dieback in North America, identifies potential processes that contribute to reduced vigour and dieback of aspen stands, and examines the scales (stand, ecosite, regional) at which these processes operate. Many factors including pathogens, nutrition, or successional changes may be involved in the decline of aspen vigour and thereby contribute to the dieback process. However, insect defoliation, drought, and thaw–freeze events appear to be the most likely factors initiating dieback in mature aspen stands. Further study is clearly needed to elucidate the mechanisms and landscape patterns of dieback. Information needs related to identifying processes and modeling landscape patterns of dieback are indicated.