General considerations as regards the human language faculty come into play when analysing adult second language acquisition. Grammar as conceived in the Principles and Parameters theory is a modularly organised system which relies on both the autonomy and the inter-relation of its subcomponents. One such inter-relation lies at the heart of this study, namely, the relation between the lexicon and the syntax. Following current assumptions it is expected that the acquisition of functional categories plays a crucial role in grammar development. However, it is a matter of debate whether language acquisition in adulthood relies on the same learning mechanisms as in childhood. The critical evaluation of this controversy leads to the conclusion that traditional learning concepts need to be revised in more dynamic terms. The dynamic approach proposed is applied in the investigation of the intra-individual variation in adult L2 German.