Now in its second edition, this Handbook is a current overview of Second Language (L2) research, providing state-of-the-art synopses of recent developments in each sub-area of the field, and bringing together contributions by emerging scholars and experts in Second Language Acquisition (SLA). Since the first edition, broad socio-political movements, alternative views of bilingualism, emergence of global markets, vast expansion of electronic resources, the development of social media, and the availability of big data have transformed the discipline, and this edition has been thoroughly updated to address these changes. It is divided into six main parts: Part I situates SLA in terms of research and practice; Part II explores individual cognitive, age-related and neurolinguistic similarities and differences; Part III outlines external, sociocultural, and interactive factors; Part IV presents profiles of bilinguals who take differing paths of acquisition; Part V describes interlanguage properties; and Part VI comprises clear models of L2 development.