As there are a lot of approaches and projects within the area of adaptive e-learning, several theoretical models have been developed in the last 15 years. Against this background, the AdeLE team implemented a first prototype on basis of informal descriptions given by literature research as well as concrete circumstances of the research project. Nevertheless, a theoretical model fulfilling certain requirements for our approach is still missing. This paper points out possibilities and limitations of existing models in the scope of the AdeLE project. Further, the authors own attempt towards formalising adaptive behaviour in e-learning environments is denoted.