The contributions of instructional design to cognitive science are discussed. It is argued that both sciences have their own object of study, but share a common interest in human cognition and performance as part of instructional systems. From a case study based on experience in teaching introductory computer programming, it is concluded that both sciences may reciprocally influence each other. Cognitive science often conducts research on cognitive processes in "silent" instructional strategies, so that guidelines for improving instructional systems are limited to those strategies. Instructional design conducts research on "spoken" instructional strategies, and observations of students working according to those strategies may provide evidence for the importance of particular cognitive processes that are neglected in current cognitive theories. However, these implications are also limited because they cannot lead to detailed descriptions of those cognitive processes. With regard to the tangent plane between cognitive science and instructional design (such as the field of intelligent tutoring systems), it is argued that these sciences must work together to reach their common goals. The ACT theory of skill acquisition is included in the discussion. (Author/SLD)