This state-of-the-science handbook offers a comprehensive discussion of input processing in second language acquisition. The volume assesses past and current research on input processing and engages the reader in critical reflection about the current state of the field and what lies ahead for future research, theory-building, and implications for language instruction. The handbook considers multiple theoretical perspectives, pivotal research findings, issues in research methodology, and instructional implications that underscore the centrality of input processing in second language acquisition. Whereas to date most research in this area has focused on input processing as it relates to the acquisition of morphosyntax and lexis, the present volume also attends to more recent theoretical advances regarding other linguistic subsystems, such as phonology and pragmatics, as well as processing resource allocation during multilevel input processing. Thorough and forward-looking, this volume is an indispensable resource to scholars and advanced students of second language acquisition, bilingualism, applied linguistics, cognitive science, psychology, and education.