The main objective of this thesis is to describe the effect on cognition of the structure of CAL simulation programs used, in science teaching. Four programs simulating a pond ecosystem were written so as to present a simulation model and to assist in cognition in different ways. Various clinically detailed methods of describing learning were developed and tried including concept maps which were found to be sammative rather than formative descriptions of learning, and to be ambiguous) and hierarchical structures (which were found to be difficult to produce). Fran these concept maps and hierarchical structures I developed my Interaction Model of Learning which can be used to describe the chronological events concerned with cognition. Using the Interaction Model, the nature of cognition and the effect that CAL program structure has on this process is described. Various scenarios are presented as a means of showing the possible effects of program structure on learning. Four forms of concept learning activity and their relationship to learning valid and alternative conceptions are described. The findings from the study are particularly related to the work of Driver (1983), Marton (1976) and Entwistle (1981).