"The research reported in this dissertation is directed towards enhancing high school students' understanding of science in an attempt to dispel a widely reported disenchantment with science subjects. An investigation was conducted into the implementation of a strategy designed to extend students' meaningful learning in science by optimising their existing cognitive abilities. The intention was that this optimisation might ultimately result in the further development of these cognitive abilities. The research was grounded in Piaget's epistemological theories of knowledge construction. These theories afford a credible means of making inferences as to possible mental processing which might be occurring during knowledge construction on the basis of observable performances. Von Glasersfeld's theory of radical constructivism which is significantly influenced by Piaget's genetic epistemology, was chosen as the basis for devising appropriate teaching strategies that constituted a practical classroom application of Piaget's crucial ideas. A five month intervention program consisting of activities based on radical constructivism was developed. Using groups of grade 9 students from the author's school, measurements were made of these students' cognitive abilities and levels of understanding of science concepts, both prior to, and after, they had experienced the intervention program. Similar measurements were obtained from equivalent groups of control students who did not experience the intervention program. The experimental students' ongoing classroom performances were recorded during the intervention period and their attitudes towards the program were assessed by means of a questionnaire. In all, data were obtained from different cohorts of students over a six year period. In comparison to the control groups, the experimental students made measurable gains in their understanding of science concepts. However no measurable gains in cognitive ability levels were reported for either experimental or control groups of students. Whilst it was noticeable that cognitive abilities constrained students' level of science understanding, the gap between these two attributes had narrowed significantly for the experimental students by the end of the intervention period. The extent of changes to experimental students' cognitive abilities and levels of science understanding appeared to be independent of their initial cognitive stages of development. However, observations of individual student's classroom performances suggested that those whose thinking operated at the lower cognitive stages of development tended to participate less effectively in the intervention activities. Teaching and learning strategies based on a radical constructivist pedagogy appear to have been successful in maximising the use of students' existing cognitive abilities and in facilitating the construction of more sophisticated understandings of their science classroom experiences. The reported lack of any general development of cognitive abilities is possibly attributable to either of two factors. Firstly the time span of the intervention program was relatively short in comparison with similar studies. Secondly, students whose thinking operated at pre-formal stages of cognitive development were observed to engage less effectively in cognitive conflict resolution, a key aspect of the intervention program designed to facilitate conceptual change and identified by Piaget as promoting cognitive development. Further research is needed in order to establish the validity of these two possible explanations.