Increasingly, children grow up hearing two languages from birth. This introductory textbook shows how children learn to understand and speak those languages against the backdrop of their language learning environments. A narrative around the bilingual development of four young children with different language profiles helps to explain the latest research findings in a lively and accessible manner. The narrative describes how bilingually raised children learn to understand and use sounds, words and sentences in two languages, and how they are able to use each of their languages in socially appropriate ways. Positive attitudes towards bilingual development from the people in bilingual children's environments and their recognition that child bilingualism is not monolingualism-times-two are the main ingredients ensuring that children grow up to be happy and expert speakers of two languages.