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 field theory, and (III) quest ions of structure formation. I have given special emphasis to the new observations of the anisotropies of the cosmic microwave background, and attempted to explain their importance for cosmology as well as for structure formation models. There have also been improved measurements in almost every cosmologically relevant field, from the Rubble constant to element abundances and galaxy distribution statistics. Quite surprisingly, the standard cosmological models can still accommodate all these new obser­ vations.