The main focus of this thesis is to investigate the mental worlds of intellectually disabled people. It is intended to provide information about how members of this population construe their environments and how recent changes in the philosophy of care have affected their construct systems. Personal construct theory is used as the model that underpins the studies in the thesis and a modified version of repertory grid technique is developed and used to explore physical and social aspects of each subject's environment. After a pilot study was conducted to establish the viability of using modified rep grid techniques with this population, a longitudinal study over a four year period investigated the social constructs of 15 intellectually disabled residents. Eight were still in an institution at the end of the study and seven had moved into the community during that period. A comparison group of eight staff were sampled at the beginning of the longitudinal study. Information is made available concerning the size and complexity of each subject's construct system. It was found that the size and content of the construct systems of intellectually disabled people is limited relative to the comparison group and does not change significantly over four years. construct systems were analysed using two computerbased programs that solved the patterns of interrelationships and a graphic presentation of the network of significant correlations between constructs was completed. It was found that the graphic presentation was adequate for the intellectually disabled respondents but not for the comparison group. No difference was found between the community-based group of intellectually disabled people and those still resident in the hospital after four years. A further study with 17 intellectually disabled people, parents and non-parents, found no difference in their construct systems of children. These results are discussed in the context of the present philosophy and practice of normalisation and social role valorisation.