Multiverse theories are physical theories according to which we have empirical access only to a tiny part of reality that may not be representative of the whole. The idea that we might live in a multiverse is often suggested as a response to the finding that various parameters seem fine-tuned for life. The combination of string theory and inflationary cosmology is taken to propose a specific implementation of the multiverse idea: the landscape multiverse. I review these lines of thought and outline the structure of the book. I also highlight its central theses and ideas.