Schemas are major knowledge structures influencing the way people acquire and store information. Sandra Marshall explores a new theory of schema development and studies the applicability of the theory as a unified basis for understanding learning, instruction, and assessment. The theory’s prescriptions for teaching are direct, and its application to assessment suggests new directions for tests. After examining the roots of the theory in earlier work by philosophers and psychologists, the author illustrates the main features of her theory with experimental evidence from students who are learning to recognize and solve arithmetic story problems. She describes individual performance with traditional empirical studies as well as computer simulation. The computer simulation reflects a new approach in modeling cognition. Marshall’s model links neural networks with symbolic systems to form a hybrid model that uses pattern matching of sets of features as well as logical step-by-step rules. Educational researchers and psychologists as well as curriculum developers will be interested in the new approaches of this work.