Robert Gagne's hierarchical learning theory categorizes learning into five types: intellectual skills, cognitive strategies, verbal information, attitudes, and motor skills. Intellectual skills are further divided into eight subcategories that increase in complexity: signal learning, stimulus-response learning, chain learning, verbal association learning, multiple discrimination learning, concept learning, rule/principle learning, and problem solving learning. These categories build upon each other, with higher-order skills requiring mastery of lower-level skills and more previous learning.