Abstract As American trademark law has expanded to cover non-traditional designators, the law’s “functionality” doctrine has arguably become the most important bulwark against overly broad trademark rights in the U.S. While the law has a stable analytic framework for “utilitarian” functionality, the same cannot be said of “aesthetic functionality,” i.e., the notion that some product features are so aesthetically pleasing or trigger such specific mental responses among consumers that those features should not be monopolized by one competitor through trademark rights. This chapter explores the aesthetic functionality doctrine in American trademark law, proposing that the most convincing cases for aesthetic functionality are really about consumers’ cognitive and psychological responses, not aesthetics. The chapter proposes that aesthetic functionality should bar trademark protection only for product features related to specific cognitive, perceptual, or aesthetic biases that were widespread among consumers before the trademark owner began its own marketing efforts.