How does the law decide when a party should face trademark liability for infringement committed by someone else? The law of secondary trademark liability addresses that question. It offers trademark law’s perspective on what the US Supreme Court has described as the “problem of identifying the circumstances in which it is just to hold one individual accountable for the actions of another.”1 This apparently straightforward question masks a complex set of underlying issues, including a surprisingly tricky one: what is direct trademark infringement? In other words, what distinguishes principal from secondary liability in trademark law? Only after defining primary infringement can the law resolve the topic of this chapter: how to define the liability of individuals who have not themselves infringed, but nonetheless should be held to account for another’s infringement.