This dissertation is the first extensive research into Amazon Patent Evaluation Express (APEX), a system Amazon created to adjudicate patent infringement disputes between patent owners and the platform’s third-party sellers. I uncover APEX’s rules, showing that it is distinct from platforms’ existing copyright and trademark enforcement mechanisms. I also explore Amazon’s motives in creating such a system, discussing legal and non-legal incentive considerations surrounding the general problem of patent infringement complaints. This dissertation then surveys federal court cases involving motions to enjoin APEX, illustrating how parties are challenging Amazon’s private adjudication as a practice. I conclude that fundamentally, policy discussion about APEX’s problems and how to fix it remains the more direct way of improving the system. This dissertation places APEX within the context of platforms increasingly acting as “lawmakers” and “courts” in our society, calling attention to a novel development in both patent law and dispute resolution.