This study seeks to blend the rigorous description of four Nigerian languages with theoretical insights. Four main tasks are involved. First, constructions involving the interface of morphology and syntax in the four languages are presented with regard to the syntax of substantives and functional categories, the morphology of functional heads and the relation between functional heads and the syntactic level of language. Secondly, these constructions are described and analysed within the framework of the Principles and Parameters Theory. Thirdly, those theoretical approaches within the Principles and Parameters Theory that can serve as tools for the analysis of the four languages are refined and modified, thereby establishing a version of the theory which may also serve for the morpho-syntactic analysis of related languages. Finally the syntactic model of functional categories is combined with a strictly morpho-semantic model of functional categories.