A Practice Note discussing single-brand markets and aftermarkets, and how to define those product markets and determine market power under Sections 1 and 2 of the Sherman Act. This Note discusses the limitations on single-brand market and aftermarket definitions and how courts have approached analysis of claims involving those markets. This Note also addresses antitrust concerns that can arise with aftermarket repair restrictions and policy and enforcement efforts to address them, such as right-to-repair laws.