Accommodating learning styles in adaptive educational systems represents an important step towards providing individualized instruction. The paper summarizes the main results reported in the literature, as well as the main criticisms of the current approaches. Thus, most of the adaptive educational systems to date are based on a single learning style model and the diagnosis is done explicitly, by means of a psychological questionnaire. The lack of an integrative, comprehensive learning style model and the questionable validity and reliability of the measuring instruments are two important problems of this approach. In this context, we advocate the use of a unified learning style model, which exhibits the following properties: i) it integrates the most relevant characteristics from several models in the literature; ii) it includes e-learning specific aspects (technology related preferences); iii) it is stored as a set of learning characteristics, not as a stereotyping model. Moreover, the student diagnosis is to be done implicitly, based on analyzing her/his interactions with the system.