Many theoretical linguists and acquisition researchers believe that acquisition data can enrich the data basis for linguistic investigations both quantitatively and qualitatively. When concepts of under specification are combined with linguistic analyses of the German Determiner Phrase (DP), they lead to different predictions for DP development. This chapter analyzes an extensive corpus of longitudinal and cross-sectional data from seven monolingual German children with respect to the predictions derived from the three under specification approaches. It argues that children are equipped with an inventory of potential grammatical features. During language development, these features function as predispositions for the categorization of syntactic and morphological elements. The chapter investigates the development of possessive constructions, and shows that the interpretation of the child language data depends on the linguistic analysis assumed for these constructions. The observation that different nominal functional elements appear at different times in development, confirms the prediction which the Lexical Learning hypothesis makes for the course of development.