This book accounts for the early stages of first language acquisition within the Principles and Parameters framework. The main arguments concern the nature of early grammars in relation to the constraints that Universal Grammar imposes on them and in relation to the lack of parameterisation which, in turn, is based on the absence of the categories responsible for crosslinguistic variation, namely the set of functional categories. The argument regarding the absence of parameterisation gives rise to important predictions insofar as similarities across languages at the early stage of development are concerned. Accordingly, the theory is tested against acquisition data from a number of languages: English, French , Greek, German, Spanish, and Irish.