This chapter introduces the functions of knowledge representation and presents a critical review of the model-based tools for assessment: Pathfinder, ALA-Reader, jMAP, HIMATT, AKOVIA. For each model-based tool, foundations and applications are discussed. Next, the tools are compared in order to illustrate their advantages and disadvantages, strengths, and limitations. The latter part postulates that there is no easy and no complete way to integrate any of the model-based tools. However, the strength of good research lies in a best possible integration: Multiple perspectives on the same construct are usually needed. Thus, the further development of existing tools as well as of new ones is necessary to explore human knowledge, its change, decision-making, performance, and problem-solving as our understanding of those complex human potentials evolves.