This book is devoted to one of the most relevant problems of modern cosmology: the formation of structures in the framework of big bang cosmology. The standard theory of gravitational instability has met with great success but has also encountered significant difficulties. In this book the alternative possibility offered by topological defects is explored in detail. A pedagogical introduction to the problem is given and several theoretical aspects of the problem are reviewed. Special emphasis is placed on the observable consequences of the presence of topological defects, and in particular...