All L2 acquirers, by definition, possess complete knowledge of an L1, and often knowledge of other languages, when they begin learning the additional one. Many of them, indeed, will begin acquiring their L2 after many years of being able users of another language. Thus, previous language knowledge is an important source of influence on L2 acquisition, and this holds universally true of all L2 learners. This chapter offers a synthesis of what we currently know about the following question: If knowledge and capabilities for competent language use are already available to L2 learners through the mother tongue and other languages they may know, how do they affect the development of the new language?