Understanding the way in which large-scale structures like galaxies form remains the most challenging problem in cosmology today. This text provides an up-to-date and pedagogical introduction to this exciting area of research. Part 1 deals with the Friedmann model, the thermal history of the Universe, and includes a description of observed structures in the Universe. Part 2 describes the theory of gravitational instability in both the linear and non-linear regimes. This part also includes chapters on the microwave background radiation, large-scale velocity fields, quasars and high redshift objects. Part 3 covers inflation, cosmic strings and dark matter. Each chapter is accompanied by a comprehensive set of exercises to help the reader in self-study. The book will be of interest to research and graduate students in cosmology, relativity, theoretical physics, astrophysics and astronomy. It is also suitable for use as a graduate textbook for introductory graduate level courses.