Theoretical cosmology has undergone a stage of rapid development since the 1990s. Given an initial theory for the origin of density fluctuations, it is now possible to make high-precision quantitative predictions of cosmic microwave background anisotropies and matter power spectra. It is hard for young scientists who want to work in the field of modern cosmology to find books which provide them with a pedagogical and comprehensive overview of recent developments. The aim of the present volume is to fill this gap. This volume contains a selection of review articles written by international experts and aims to give the reader a comprehensive introduction to modern cosmology. It goes beyond existing books and conference proceedings in providing the reader with an in-depth survey of recent advances in modern cosmology. Some of the topics (e.g., the modern theory of CMB anisotropies and their application to defect models) are presented for the first time in a way suited for physicists who want to start working in the field or gain a solid foundation in modern cosmology. The review article on alternative ideas in cosmology which might address some of the problems of the present models is also a useful feature of this book.