The aim of this study was to investigate sentence production deficits in subjects with aphasia, with a view to improving the description of the observed features of performance and determining the nature of the underlying impairment. An analysis of narrative speech was designed which described sentence production in terms of thematic, phrasal and morphological structure. The comprehensive analysis procedure allowed the sentence production of non-fluent aphasic subjects, fluent aphasic subjects and normal control subjects to be compared. The results of the narrative analysis questioned the validity of grouping subjects via the fluency of their speech; there was extensive variability within each group and the deficits seen in the nonfluent and fluent subjects were not differentiable. Garrett's (1980) model of normal sentence production provided a more beneficial framework for characterising sentence production deficits in aphasia. The majority of the subjects with aphasia presented with a combination of functional and positional level deficits. Selective deficits were, however, identified in the production of thematic structure, complex phrases, function words and inflectional morphology. The independence of functional and positional level processing was confirmed by an additional study of narrative speech investigating how thematic structure influenced subsequent phrasal realisation. There was no trade-off between the complexity of the predicate argument structure (in terms of the number of phrasal components associated with the verb) and the complexity of the phrases used to realise those arguments. In addition, the argument status of the phrase was not found to influence its complexity. The number of phrasal components in an utterance and the complexity of those phrases was only influenced by the information to be conveyed. The narrative analysis allowed the likely location of a subject's impairment to be identified. An investigation of four subjects with apparent difficulties in producing the functional level representation found that differential deficits were responsible for their production of thematic structure. These results provide support for the three subprocesses suggested by Schwartz (1987): - the retrieval of semantic information, the creation of the predicate argument structure and the assignment of thematic roles to lexical items.