Analogy, conceptual change and problem reformulation have been central components in the exploration of human problem solving. A Situation Theoretic approach is developed to model analogy and conceptual change. This model is then used to relate a problem's representation to the associated cognitive difficulty. In this Unified framework the cognitive difficulty of isomorphic problem situations is defined in terms of the task, objects and relations of the problem situation. These compo?nents are then decomposed based on an Ecological Information Processing user model. The decomposition turns a problem sit?uation the structure and dynamics of the problem; the rules or constraints which are applicable; and the necessary instruc?tions for user interaction. From this, the cognitive difficulty associated with a problem representation is shown to be largely determined by the "instructional" component.