This volume aims to provide a broad view of second language acquisition within a comparative perspective that addresses results concerning adult and child learners across a variety of source and target languages. It brings together contributions at the forefront of language acquisition research that consider a wide range of open questions: What are the precise mechanisms underlying acquisition? How can we characterize learners’ initial state and predict their degree of final achievement? What role do specific (typological) properties of source and target languages play? How does fossilization occur? How does the relative complexity of cognitive systems in adult and child learners affect acquisition? Does language learning influence cognitive organization? Can language learning shed light on our general understanding of human language and language processing?