One major school of quantum theory posits a multiplicity of universes; but what does that imply about the reality we live in? A simple experiment, familiar to every student of physics, involves light passing through slits in a barrier; its results, according to Oxford physicist Deutsch, lead inevitably to the idea that there are countless universes parallel to our own, through which some of the light must pass. This "many worlds'' interpretation of quantum theory has gained advocates in recent years, and Deutsch argues that it is time for scientists to face the full implications of this idea. (After all, the entire point of science is to help us understand the world we live in--the "fabric of reality'' of his title.) To that end, he outlines a new view of the multiverse (the total of all the parallel universes), combining ideas from four "strands'' of science: quantum physics, epistemology, the theory of computation, and modern evolutionary theory. He argues that quantum computation, a discipline in which he is a pioneering thinker, has the potential for building computers that draw on their counterparts in parallel universes; this could make artificial intelligence a reality, despite Roger Penrose's objections (which Deutsch deals with in some detail). Likewise, time travel into both the future and the past should be possible, though not in quite the form envisioned by science fiction writers; the trips would almost certainly be one-way, and they would likely take the travelers into different universes from the one they began in. Deutsch takes particular pains to refute Thomas Kuhn's "paradigm'' model of science, which essentially denies progress. A final chapter looks at the long-range implications of his views, including the place of esthetic and moral values (areas more scientists now seem willing to confront). Not easy going by any means, but worth the work for anyone interested in the thought processes of a scientist on the leading edge of his discipline.