This article describes an approach to modifying science students' alternative conceptions of physical phenomena by means of diagnostic and remedial microcomputer programs designed in accord with a model of conceptual change. Current applications of microcomputers are discussed, including numerical work, drill and testing, tutorials, simulation, and real time use. A review of the literature on alternative conceptions focuses on the concepts of speed and force in physics instruction, and a model of conceptual change is outlined which implies that instruction should involve the identification or diagnosis of student conceptions, the lowering of the status or remediation of alternative conceptions, and the raising of the status of the new instructional content. Two specific programs designed to diagnose some alternative conceptions of speed and force are described, as well as the results of using the programs with first year university science students. These results indicate that the programs effected dramatic changes in a common alternative conception of speed and were able to diagnose several interrelated aspects of alternative conceptions of force. Twenty-two references are listed. (Author/LMM)