[Charu N. Gupta] Abstract: Economists’ standard notion of intellectual property rights considers a single patent per product, with a clearly defined scope, certain enforcement, and a fixed term of monopoly protection. Yet common across industries are “imperfect” intellectual property rights: More than one patent may cover a single product, with the scope and enforcement of each uncertain, contributing to an indeterminate period of monopoly protection. Using data on the pharmaceutical industry, I systematically document the presence of imperfect intellectual property rights and provide the first evidence on the extent to which they impact competition.