This thesis investigates the nature of the interface between two components of language - morphology and phonology - in children with Grammatical-Specific Language Impairment (G-SLi), compared to those with typically-developing language. I focus principally on the impact of phonological complexity on past tense inflection, but I also investigate other areas of rnorpnotogy. More specifically, I show that for G-SLI children:- There exists a phonological impairment that is independent of morphology. This impairment is characterised by the simplification of complex syllable structure, and by syllabic and segmental errors when the word starts with an initial unstressed syllable. There exists an impairment in past tense morphology, characterised by suffix omission, that is independent of phonology. Phonological factors affect past tense morphology. Specifically, suffix omission rates are higher when inflection (i) creates clusters at the word-end or (ii) requires the syllabic allomorph lidJ. Phonological factors also affect plural and present progressive formation. Unlike past tense morphology, derivational morphology is not subject to suffix omission. However, non-target derivational forms result when stimuli are morphologically or phonologtcally complex. I argue that grammar has a modular structure, and I propose that deficits in one or more of the following modules - syntax, morphology and phonology - can impact on past tense inflection. This model, termed the 'Computational Grammatical Complexity* (CGC) hypothesis, can account for why tense is an area of exceptional difficulty for children with SLI. This investigation is underpinned by a rigorous theoretical framework. Not only does using a cognitive scientific and linguistic framework further our understanding of the nature of the deficit in SLI, but SLI provides a valuable testing ground for theories of language acquisition and the representation of language in the brain.

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A number of researchers have investigated how vocabularies are organised in terms of their depth. This work provides only a partial picture of vocabulary organisation, however, because of the limited aspects of knowledge analysed, and the testing methods employed. There are no standardised tests of vocabulary organisation, and this lack has undoubtedly contributed to the inconclusive results of studies in this area. What vocabulary researchers need is a valid and reliable test of vocabulary organisation. This thesis examines some of the problems which need to be confronted in the development of a tool of this sort. The thesis described a test of Lexical Organisation (LOT) in which test takers are asked to select the strongest collocation in a set of three words. This test was able to distinguish native speakers from non-native speakers, but it was not sensitive enough to distinguish learners at different levels of proficiency. A revised version of LOT, the Two-choice LOT (TCLOT), was more successful in this regard. TCLOT proved to be a reliable test, but its validity as a test of vocabulary organisation was not fully confirmed. The thesis considers the implications of these results for theories of vocabulary size and organisation, and for models of vocabulary which attempt to describe vocabulary knowledge in terms of a small number of orthogonal dimensions.