This thesis presents a theory of the early stages of first language acquisition. Language is
characterised as constituting an instructional environment - diachronic change in language
serves to maintain and enhance sources of structural marking which act as salient cues that
guide the development of linguistic representations in the child's brain. Language learning is
characterised as a constructivist process in which the underlying grammatical representation
and modular structure arise out of developmental processes. In particular, I investigate the
role of closed-class elements in language which obtain salience through their high occurrence
frequency and which serve to both label and segment useful grammatical units. I adopt an
inter-disciplinary approach which encompasses analyses of child language and agrammatic
speech, psycholinguistic data, the development of a developmental linguistic theory based on
the Dependency Grammar formalism, and a number of computational investigations of
spoken language corpora. I conclude that language development is highly interactionist and
that in trying to understand the processes involved in learning we must begin with the child
and not with the end-point of adult linguistic competence