This study investigates the relevance of contemporary theories of literacy and associated practices for the assessment of functional reading and writing in acquired aphasia. `Ideological' and `autonomous' models of literacy are described. The ideological model represents reading and writing as a set of activities intrinsically connected with social behaviour. The autonomous model represents literacy as a set of discrete cognitive and linguistic skills which can operate regardless of context. An analysis of currently-used aphasia assessments, and the traditions from which they spring, suggests that they operate a predominantly autonomous model of literacy. Theoretical analysis is reinforced by three studies of people with and without aphasia. The first study demonstrates the diversity of everyday literacy practice of 50 non-aphasic people. The second study is a qualitative investigation of the literacy practices of twenty people with mild-moderate aphasia. Again, a diversity of psychosocial factors is found to affect literacy practice. These concern the roles both lost and taken on by the aphasic respondents, and the ways in which they cope with their condition. In the final study of the reading and writing difficulties of three aphasic people, a direct comparison is made of autonomous and ideological approaches to aphasia assessment, using a cognitive neuropsychological battery of tests, and an investigative procedure developed along ethnographic principles. These qualitative data suggest that the ideological approach addresses functional use of written language more adequately than the autonomous approach. There is little predictive relationship between the two approaches. The relevance of an ideological approach is supported, and the qualitative methodology associated with this model endorsed as a means of investigating aphasia. While the contributions of autonomous measures to the knowledge of impairment are acknowledged, the shortcomings of prescriptive approaches to functional assessment are also apparent. Contemporary theories of disability contribute to a re-evaluation of current approaches in the study of aphasia. It is argued that the personal interpretation of impairment must be addressed in functional assessment. The study represents a development in the theory of functional assessment of aphasia. It also develops a methodology which has implications for clinical practice.