Personalised e-Learning represents a major step-change from the one-size-fits-all approach of traditional learning platforms to a more customised and interactive provision of learning materials. Adaptive learning can support the learning process by tailoring learning materials to individual needs. However, this requires the initial preparation of content upfront, which is a laborious task--and organizations have to target their limited resources effectively. In order to guide the process of creating adaptive learning materials, the criteria for adaptation--or adaptation needs--have to be known. The aim of this paper is to identify these adaptation criteria, applying a mixed method procedure. First, thirty adaptive systems selected from the literature are investigated using a qualitative content analysis. Then, the resulting set of adaptation criteria is validated by experts in the form of a series of two online questionnaires. As a result, a set of 13 adaptation criteria representing different adaptation needs emerge.