Fourteen years is a long time, and especially in the field of cosmology new observational results and new theoretical ideas seem to appear at a steadily increasing rate. It is achallenge to try to review the current status, to give a reasonably fair account of new developments, and not to increase the size of the book out of all proportion. So this fourth edition is practically a new book, with many chapters and sections newly written, not just updated. I have kept the original layout of the book with three parts concerned with (I) the standard model, (II) some basic implications of quantum...