This book provides a theory of first language acquisition in the syntactic framework of the theory of Universal Grammar. It addresses issues related to the earliest stage of development which ends roughly around the child's second birthday. The theory put forward capitalises on the traditional observation that early child grammars characteristically lack lexical and morphological elements which belong to the 'closed-class' system. This book provides an account of the grammatical differences between the set of functional categories and the substantive categories.