The language experience of children developing in linguistically diverse environments is subject to considerable variation both in terms of quantity and quality of language exposure. It is an open question how to investigate language exposure patterns and more important which factors are relevant for successful language learning. For example, children acquiring a minority language, including a signed language, are exposed to less variety of input than children acquiring a more global language. This is because they are living in a smaller linguistic community and with fewer occasions to use the language in everyday life. Despite this reduced input, most native signers are successful language learners. In contrast native language competence is not always achieved in signing deaf children with hearing parents or those with cochlear implants learning a spoken language. A similar outcome but with very different reasons has also been reported for hearing children with language impairment. In these populations acquisition of morphosyntactic aspects is developing atypically ending with an uncomplete linguistic repertoire. The circumstances of exposure during language development tend to differ in significant ways with respect to a large number of factors, such as, (i) length, quality and quantity of input, (ii) social status and attitudes toward the language, (iii) cognitive abilities required for language learning, and (iv) age of first exposure. Having early exposure to a range of different speakers is important in the acquisition of any language and may affect language proficiency. However, negative societal attitudes or a cognitive based disadvantage may create an unfavourable learning environment that prevents language learning from surfacing typically. This situation inevitably generates a different type of exposure for the child and consequently different language competence. In this Research Topic we intend to encourage the debate on social, linguistic and cognitive factors at play for designing an effective environment for language acquisition aiming at integrating linguistic variables coming from theoretical studies on language with environmental variables, such as, measures of language input or cognitive abilities on functions ancillary to language development.

Ever since attempts were made to describe and explain normal language development, references to exceptional circumstances have been made. Variations in the conditions under which language is acquired can be regarded as natural experiments, which would not be feasible or ethical under normal circumstances. This can throw light on such questions as: *What language input is necessary for the child to learn language? *What is the relationship between cognition and language? *How independent are different components of language function? *Are there critical periods for language development? *Can we specify necessary and sufficient conditions for language impairment? This book covers a range of exceptional circumstances including: extreme deprivation, twinship, visual and auditory impairments, autism and focal brain damage? Written in a jargon-free style, and including a glossary of linguistic and medical terminology, the book assumes little specialist knowledge. This text is suitable for both students and practitioners in the fields of psycholinguistics, developmental and educational psychology, speech pathology, paediatrics and special education.

How and why do all children learn language? Why do some have difficulties while others are early language learners? What are the consequences of early bilingualism? Is it possible to reach native-like competence in a foreign language? Although we still cannot fully answer these questions, research during the last two decades has begun to solve some pieces of the puzzle. This book proposes an interdisciplinary collection of writings from some of the best specialists across several fields in cognitive science, offering a wide sample of recent advances in the study of first language acquisition, bilingualism, second language acquisition, and disorders of oral language. It is addressed to all researchers and students interested in language acquisition, as well as to teachers, clinicians and parents, who will find therein many new findings and varied methodological approaches, as well as challenging questions that are still debated and in need of further research.

Language acquisition is a human endeavor par excellence. As children, all human beings learn to understand and speak at least one language: their mother tongue. It is a process that seems to take place without any obvious effort. Second language learning, particularly among adults, causes more difficulty. The purpose of this series is to compile a collection of high-quality monographs on language acquisition. The series serves the needs of everyone who wants to know more about the problem of language acquisition in general and/or about language acquisition in specific contexts.

A clear and practical introduction to second language acquisition, written for students encountering the topic for the first time.