This paper presents a comprehensive conceptual framework and notation for learner modelling in intelligent tutoring systems (ITS). The framework is based upon the computational distinction between behavior, behavioral knowledge, and conceptual knowledge (in a “vertical” dimension) and between the system, the learner, and the system's representation of the learner (in a “horizontal” dimension). All existing techniques for learner modelling are placed within this framework. Methods for establishing the search space for learner models and for carrying out the search process are reviewed. The framework makes clear where particular learner modelling techniques are focused and shows that they are often complementary since they address different parts of the framework.