Both Vygotsky, a psychologist, and Halliday, a social linguist, argue for the central role of language in human development. Language is the principal mode of meaning making; it mediates both the communication through which thinking with others is made possible and also the inner speech through which individual thinking is brought under conscious control. I develop this central claim by drawing on the writings of Vygotsky and Bakhtin, as well as those of their colleagues and followers to argue for the importance of encouraging dialogue in all educational settings in order to enable learners of all ages to construct knowledge together, and thereby to enhance their individual understanding of the world and their potential for action in it. (Contains 1 figure.)