The ability to speak two or more languages is a common human experience, whether for children born into bilingual families, young people enrolled in foreign language classes, or mature and older adults learning and using more than one language to meet life's needs and desires. This Handbook offers a developmentally oriented and socially contextualized survey of research into individual bilingualism, comprising the learning, use and, as the case may be, unlearning of two or more spoken and signed languages and language varieties. A wide range of topics is covered, from ideologies, policy, the law, and economics, to exposure and input, language education, measurement of bilingual abilities, attrition and forgetting, and giftedness in bilinguals. Also explored are cross- and intra-disciplinary connections with psychology, clinical linguistics, second language acquisition, education, cognitive science, neurolinguistics, contact linguistics, and sign language research.