This chapter focuses on the acquisition of English as a mother tongue among English-speaking children. With the exception of very few people, everyone is proficient in at least one language, and the majority of people in the world are able to use more than one language. As fascinating as it is to study children and how they acquire language, it is notoriously difficult to get at. Whereas with adults it is possible to offer grammatical tests or ask them what they are thinking about, there is no obvious way to get into the mind of a child who has not yet acquired a mother tongue. While parents and other figures in child’s life may want to give themselves credit for training children explicitly in language, in fact explicit input is rarely very effective, especially during the early stages of language acquisition.