Differentiated instruction contends that teachers should vary their instructional strategies to match the learners' individual differences. However, this is challenging due to various constraints of classroom and contextual variables. Adaptive systems offer a solution to this challenge, especially as instruction has increasingly moved towards an online format. Specifically, adaptive systems use artificial intelligence to adapt content or interface presentation to the individual needs of the learner. A historical review suggests that the following four different approaches to adaptive systems have emerged over time: adaption to cognitive abilities, learning styles (debunked), affective states, and external environments.