How is it that we can all open our mouths and speak, often at considerable length, without consciously thinking about the construction of the sentences we are using? And how is it that four-year-old children can apparently do the same thing? The popular notion of how children come to speak their first language is that their parents teach them words, then phrases, then sentences and longer utterances. There is widespread agreement among linguists that this account is wrong, though there is much less agreement about what the correct answer is. There are, in fact, numerous ongoing debates about it. Without pressing dogmatic views about it, this text seeks to give the reader the necessary background for forming views on the main issues. It describes the theories that have been most influential during the 20th century, namely, those of Skinner, Piaget, Halliday, Chomsky and Karmiloff-Smith, as well as a great deal of research that has been done by linguists and psychologists. No previous knowledge of linguistics or psychology is assumed.