"What is real?" has been one of the key questions of philosophy since its beginning in antiquity. It is a question that, due to such films as The Matrix, has also made its way into popular culture. But it is not just a question philosophers ask. It is also asked by scientists when theyinvestigate whether the fundamental constituents of matter are actually "out there" or just a mere abstraction from a successful theory. Cognitive scientists ask it when trying to find out which set of the bewildering array of data processed by our brain could constitute the basis for suchsupposedly fundamental entities like the free agent or the self.This Very Short Introduction discusses what reality is by looking at a variety of arguments, theories and thought - experiments from philosophy, physics, and cognitive science.