This book presents the first systematic account of conformal geometry of n-manifolds, as well as its Riemannian counterparts. A unifying theme is their discrete holonomy groups. In particular, hyperbolic manifolds, in dimension 3 and higher, are addressed. The treatment covers also relevant topology, algebra (including combinatorial group theory and varieties of group representations), arithmetic issues, and dynamics. Progress in these areas has been very fast over the last two decades, especially due to the Thurston geometrization program, leading to the solution of many difficult problems. A strong effort has been made to point out new connections and perspectives in the field and to illustrate various aspects of the theory. An intuitive approach which emphasizes the ideas behind the constructions is complemented by a large number of examples and figures which both use and support the reader's geometric imagination. The text will be of value to graduate students and researchers in topology, geometry, group representations and theoretical physics.