Learning a second or foreign language is commonplace in today’s world. In fact, recent estimates (Grosjean, 2010) suggest that more than half of the world’s population knows more than one language. Some of this dual language knowledge comes from family or societal sources, that is, growing up in a dual-language family or a bi-or multilingual society; in other instances, second language knowledge comes from an instructional setting. In some instances, learning begins postpuberty; in other instances, it begins in childhood. Despite these varied facts, we are still a long way from understanding how second languages are learned, why many individuals have difficulty in reaching high levels of proficiency in a second language, or even what the best pedagogical approach might be. This Handbook of Second Language Acquisition provides comprehensive coverage of the field of second language acquisition (SLA) with an effort to incorporate a wide range of different approaches to understanding how languages are learned.