Based on the formal concept of "knowledge structures" originally proposed by Jean-Claude Falmagne and Jean-Paul Doignon, this book contains descriptions of methodological developments and experimental investigations as well as applications for various knowledge domains. The authors address three main topics: * theoretical issues and extensions of Doignon & Falmagne's theory of knowledge structures; * empirical validations of specific problem types and knowledge domains, such as sentence comprehension, problem solving in chess, inductive reasoning, elementary mathematical reasoning, and others; and * application of knowledge structures in various contexts, including knowledge assessment, intelligent tutoring systems, and motor learning. Unlike most other approaches in the literature in cognitive psychology, this book provides both a rigorous mathematical formulation of knowledge-related psychological concepts and its empirical validation by experimental data.