AutomataTutor is an online tool that helps students learn basic concepts in theory of computation, such as fi nite automata and regular expressions. The tool provides personalized feedback when students submit incorrect solutions, and also helps teachers managing large classes by automatically grading homework assignments. AutomataTutor has already been used by more than 2000 students at 12 different Universities in 4 different continents. In this paper, we summarize our experience in building such a system. We describe the algorithms that are used to produce personalized feedback, and then evaluate the tool and its features through extensive user studies involving hundreds of participants.