Seven years ago Manfred Pienemann proposed a novel psycholinguistic theory of language development, Processability Theory (PT). This volume examines the typological plausibility of PT. Focusing on the acquisition of Arabic, Chinese and Japanese the authors demonstrate the capacity of PT to make detailed and verifiable predictions about the developmental schedule for each language. This cross-linguistic perspective is also applied to the study of Li transfer by comparing the impact of processability and typological proximity. The typological perspective is extended by including a comparison of different types of language acquisition. The architecture of PT is expanded by the addition of a second set of principles that contributes to the formal modeling of levels of processability, namely the mapping of argument-structure onto functional structure in lexical mapping theory. This step yields the inclusion of a range of additional phenomena in the process-ability hierarchy thus widening the scope of PT.