A substantially revised and updated edition of the introductory textbook on cosmology for undergraduates. While designed as the main text for a course given at second- or third-year level, the book 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 is its updated epilogue of 20 controversies in cosmology today. The latest results from the WMAP mission are included and there is a stronger emphasis on the cosmological constant. The book has an extensive glossary and the exercises have been substantially expanded. A stronger emphasis on the physical basis for cosmology is included throughout.