This is the first book on the acquisition of the German case system by foreign language learners. It explores how learners in their interlanguage progress from the total absence to the presence of a case system. This development is characterized by an evolvement from marking the argument’s position to marking the argument’s actual function. Theoretically couched within Processability Theory, the book deals with the feature unification and the mapping processes involved in case marking, and critically examines previous findings on German case acquisition. Empirically, the book consists of longitudinal data of 11 foreign language learners of German, which was collected over a period of 2 years.