The paper studies some aspects of adaptation in Computer-aided learning. The three adaptation levels are described. They are adaptation to user category; adaptation to group of users; adaptation to each user. Different variations of adaptation to a student are considered. The first adaptation level provides adaptation to each class of users of computer-aided tutoring system and depends on student needs in learning process such as tutoring, testing, questions to a teacher, student forums and so on, which are described in the paper. The second adaptation level provides to a group of users inside a category. The main didactic questions “what to teach?” and “how to teach?” are discussed to realize the adaptation of the second level. An adaptation of this level depends on user age, his speciality and a program (academic or professional). The third adaptation level is based on personal characteristics of each student as well as on his previous experience, on the execution of current tasks and so on. A student model is used to provide adaptation of every level. The classification of student models is presented and characteristics of each class of models are considered. Examples of some student models used in Computer-aided learning are described. The multilevel student model, the spiral student model and the five-component student model are outlined.