One of the most important aspects of solid materials is the regularity of the arrangement of the constituent molecules, that is, the long-range order. The focus of this book is on the contribution made by the ordering of bond orientations (as distinguished from the orientations of the molecules themselves) on the behavior of condensed systems, particularly their phase transitions. Examples in which bond-orientational effects play an important role are liquid crystals, quasicrystals, and two-dimensional crystals. This book contains contributions by many of the foremost researchers in the field. The chapters are tutorial reviews of the subject, written both for the active researcher looking for a review of a topic and for the graduate student investigating an exciting area of research. The contributions include an overview by J.D. Brock, Cornell; a discussion of computer simulation studies by K.J. Strandburg, Argonne; chapters on phase transition in hexatic liquid crystals by C.C. Huang, Minnesota and C.A. Murray, Texas A & M; and chapters on quasicrystals by S. Sachdev, Yale, M.V. Jaric, A.I. Goldman, Iowa State, and T.-L. Ho, Ohio State.