This textbook introduces fundamental concepts and results in the theory of first language acquisition, bringing together linguistic phenomena on the one hand and learning and cognitive development on the other. Grammar and development are woven together through a range of case studies that provide students with the tools to think about (a) what is already understood about language acquisition; (b) what methods we use to identity children's grammatical knowledge; and (c) how to approach problems in any area of grammatical acquisition. Each of the book's five parts illustrates a foundational concept in the theory of language and its acquisition: the nature of learning in the biological world; categories in phonetics, phonology, morphology, and syntax; hierarchical structure; grammatical dependencies; and the role of information processing systems in the theory of language acquisition. Designed with advanced undergraduate and beginning graduate students in mind, the book includes discussion questions and suggestions for further reading at the end of each chapter.