This is a substantially revised and updates new edition of a classic introductory textbook. Although there are a wealth of cosmology texts at postgraduate level, Cosmology remains the classic introduction to modern cosmology for undergraduates. While designed as the main text for a course given at second or third year level, it is sufficiently self-contained for anyone with school science to understand. There is a strong emphasis on observational cosmology, with introductory chapters on the visible universe, our galaxy and other galaxies and the empirical basis for cosmological theory. After an account of the big bang model, there are chapters on the early stages of the big bang and galaxy formation. Finally, there are chapters on cosmological tests and on alternative theories. One feature of the book it its updated epilogue of twenty controversies on cosmology today. The latest results from the WMAP mission are included and a wealth of new material, including a stronger emphasis on the cosmological constant. It has an extensive glossary and the exercises have been substantially expanded. A stronger emphasis on the physical basis for cosmology is included throughout.