This paper describes the philosophy of intelligent instructional systems and presents an example of one such system in the domain of manipulative mathematics--BLOCKS. The notion of BLOCKS as a paradigmatic system is explicated from both the system development and educational viewpoints. From a developmental point of view, the modular design of BLOCKS provides a working framework within which to explore different monitoring functions and various tutoring strategies. From an educational viewpoint, BLOCKS provides a dramatic example of the potential of a computerized intelligent tutor in a laboratory environment. By monitoring the student's behavior, the system can notice interesting situations and direct the student's attention to them. In this way, the computer can provide conceptual structure and guidance to a student's otherwise undirected experiences. (Author/VT)