Understanding the way in which large-scale structures such as 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 nonlinear regimes. This part also includes chapters on the microwave background radiation, Large-scale velocity fields, quasars, and high redshift objects. Part 3 of the book covers inflation, cosmic strings, and dark matter. Each chapter is accompanied by a comprehensive set of exercises to help the reader in self-study.