An intelligent tutoring system combines artificial intelligence, cognitive science, and educational theory to support adaptive instruction. Its architecture comprises four modules: (1) the expert-knowledge module, which supplies domain knowledge and supports the generation of questions, answers, and solutions; (2) the student-model module, which tracks the learner's changing knowledge and skills; (3) the tutor module, which uses the student model to select a teaching strategy and determine what to present next; and (4) the user-interface module, through which the system and learner interact.