This book is a comprehensive and coherent introduction to the role of cosmic strings and other topological defects in the universe. After an introduction to standard cosmological theory and the theory of phase transitions in the early universe, the book then describes, in turn, the properties, formation, and cosmological implications of cosmic strings, monopoles, domain walls and textures. It concludes with a chapter considering the role of topological defects in inflationary universe models. Ample introductory material is included to make the book readily accessible. It will be of interest to graduate students and researchers in particle physics, astrophysics and cosmology.