AbstractAn improved understanding of lignocellulosic biomass availability is needed to support proposed expansion in biofuel production. Fifteen studies that estimate availability of lignocellulosic biomass quantities in in the U.S. and/or Canada are reviewed. Sources of differences in study methods and assumptions and resulting biomass quantities are elucidated. We differentiate between inventory studies, in which quantities of biomass potentially available are estimated without rigorous consideration of the costs of supply, versus economic studies, which take into consideration various opportunity costs and competition. The U.S. economic studies, which included reasonably comprehensive sets of biomass categories, estimate annual biomass availability to range from 6 million to 577 million dry metric tonnes (dry t), depending on offered price, while estimates from inventory studies range from 190 million to 3849 million dry t. The Canadian inventory studies, which included reasonably comprehensive sets of biomass categories, estimate availability to range from 64 million green t to 561 million dry t. The largest biomass categories for the U.S. are energy crops and agricultural residues, while for Canada they are expected to be energy crops and logging residues. The significant differences in study estimates are due in large part to the number of biomass categories included, whether economic considerations are incorporated, assumptions about energy crop yields and land areas, and level of optimism of assumptions of the study.