We present a brief introduction to the quantum theory of light as it is understood in the field of quantum optics. Our aim is not to review the topic, which would require a very extensive article (or even a book of several volumes) but rather to provide sufficient background to set the ideas in the following papers in their correct context.This article is part of the theme issue 'The quantum theory of light'.
Keywords: History of quantum theory; Non-clssical states of light; Photons.