The process of acquisition, and the nature and extent of retention, of a hierarchy of intellectual skills in genetics were examined for each of three adults by case study. Task materials, based on a skill hierarchy and interview protocol, were designed to promote learning with understanding. Various theoretical models and schemes were devised to identify and monitor learning processes and changes in cognitive structure of the learner. Additional information was obtained on the stability and generalizability of learning processes, and the nature and extent of retention and accessibility of learned material. Evidence was obtained for qualitative differences in learning process and outcome. Results indicate evidence for two different learning styles which involve different specific learning strategies and which differ in the extent of incorporation of new material into the learner's cognitive structure, and on the subsequent retrieval of this material. One learning style is considered particularly effective for both incorporation and retrieval. The theoretical bases developed in this study may prove useful for identifying changes in cognitive structure of, and strategies employed by, a learner as he or she progresses through similar intellectual skill learning sequences, and thus for remediation by a teacher of gaps or deficiencies in the learner's strategies or cognitive structure. This procedure should facilitate progression by the learner to a thorough understanding of the terminal skills.The interpretations of several experiments relating to air pressure are analysed using the commentaries of children aged 11 to 13 in class and interviews carried out with children of the same age.