The Supreme Court and the Federal Circuit have repeatedly emphasized thepublic interest in testing the validity of patents, weeding out patentsthat should not have been issued. But there is one important group ofpeople the law systematically prevents from challenging bad patents.Curiously, it is the very group patent law is supposed to support:inventors themselves. The century-old doctrine of assignor estoppelprecludes inventors who file patent applications from later challenging thevalidity or enforceability of the patents they receive. The statedrationale for assignor estoppel is that it would be unfair to allow theinventor to benefit from obtaining a patent and later change her tune andattack the patent when it benefits her to do so. The Supreme Court hastraditionally disfavored the doctrine, reading it narrowly. But the FederalCircuit has expanded the doctrine in a variety of dimensions, and appliedit even when the benefit to the inventor is illusory. Further, the doctrinemisunderstands the role of inventor-employees in the modern world.More important, the expansive modern form of assignor estoppel interferessubstantially with employee mobility. Inventors as a class are put underburdens that we apply to no other employee. If they start a company, oreven go to work for an existing company in the same field, they will not beable to defend a patent suit from their old employer. The result is a sortof partial noncompete clause, one imposed without even the fiction ofagreement and one that binds anyone the inventor comes in contact withafter leaving the job. Abundant evidence suggests that noncompetes ingeneral retard innovation and economic growth, and several states prohibitthem outright, while all others limit them. But assignor estoppel is afederal law doctrine that overrides those state choices.It is time to rethink the doctrine of assignor estoppel. I describe thedoctrine, its rationale, and how it has expanded dramatically in the past25 years. I argue that the doctrine is out of touch with the realities ofboth modern inventing and modern patent law, and that it interferes withboth the invalidation of bad patents and the goal of employee mobility.Should the Supreme Court take up the doctrine, it is unlikely to survive inits current form. Rather, it should – and will – return to its much morelimited roots.