A distinctive characteristic of human informal, everyday, acquisition is that it relies heavily on learning by observation of the actions of others. We believe that the normal child embarks on a course of observational learning between 12-and 24-months that accounts for his/her speed of learning and overall breadth of knowledge. By contrast, learning disabled, premature, or otherwise impaired children may not proceed with observational learning at the normal rate or in the normal ways. At present, precise knowledge of the development of observational learning is lacking, which prevents the formulation of research-based theory to account for this fundamental mode of acquisition. The objective of the proposed research is the precise and systematic study of observational learning during its formative period between 12-and 24-months. Our method will be based on deferred imitation of actions with objects that are demonstrated to children. In Exp. 1 actions will be grouped into four classes representing different domains of practical intelligence. In Exp. 2 the actions to be studied for deferred imitation will be symbolic acts that involve more abstract awareness and representation. Exp. 3 will concentrate on analytical and parametric determination of long-term memory development during the second year of life. The age period to be studied is one of rapid changes in information processing, and we expect that results will provide a rich source of qualitative as well as quantitative information about the growth of observational learning. Scientific, as contrasted with speculative, advances in understanding the development of observational learning as a ubiquitous, species-typical mode of acquisition should prove highly important for evaluating and studying the learning process in children with developmental disabilities and learning difficulties. Knowledge of the start-up period of observational learning may even provide a route to better detection of and the prevention of developmental disabilities in which the learning functions are impaired. The principal disciplines involved in this endeavor are the psychological and pediatric sciences.