The Massachusetts Institute of Technology (MIT) has invented or improved many world-changing things--radar, information theory, and synthetic self-replicating molecules, to name a few. Last month the university announced, to mild fanfare, an invention that could be similarly transformative, this time for higher education itself. It is called MITx. In that small lowercase letter, a great deal is contained. MITx is the next big step in the open-educational-resources movement that MIT helped start in 2001, when it began putting its course lecture notes, videos, and exams online, where anyone in the world could use them at no cost. The project exceeded all expectations--more than 100 million unique visitors have accessed the courses so far. Meanwhile, the university experimented with using online tools to help improve the learning experience for its own students in Cambridge, Mass. Now MIT has decided to put the two together--free content and sophisticated online pedagogy--and add a third, crucial ingredient: credentials. Beginning this spring, students will be able to take free, online courses offered through the MITx initiative. If they prove they have learned the material, MITx will, for a small fee, give them a credential certifying as much. In doing this, MIT has cracked one of the fundamental problems retarding the growth of free online higher education as a force for human progress.