Abstract: In competition law, market power requirements are often articulated in terms of market
shares. The use of market share thresholds, however, conflates two distinct questions: (1) How
much market power exists in a given situation? (2) How much market power should the law
require? As a consequence, neither question is answered, or even directly illuminated.
Furthermore, because market shares are not themselves measures of market power but instead
merely a factor that bears on its magnitude in a given setting, they are inapt answers to both
inquiries. Their use involves a category mistake. The identified problems are illustrated by
unpacking Learned Hand’s famous pronouncement in Alcoa of the market shares required for the
offense of monopolization, but the core defects characterize all market share declarations.