Hardly any term in implant surgery has been misused as often as compromised bone. Most attempts to shed light on the concept have been confined to data on changes in bone microstructure, while a comprehensive review of the cellular, microstructural, and gross anatomic pathology and physiology; current diagnostic tools; and treatment options is still not available. Few have attempted to go beyond the four well-known bone quality classes defined by Lekholm and Zarb in 1985, which continue to be the basis of discussions in the literature. One notable exception is Aging, Osteoporosis, and Dental Implants, edited by G. Zarb, U. Lekholm, T. Albrektsson, and H. Tenenbaum (Quintessence, 2002). Although, as the title suggests, the book is focused specifically on age-related changes and their impact on dental implants, contributing to this work inspired me, along with my colleagues at the University of Vienna, to review the problem of compromised bone and its impact on dental implants in a more comprehensive way and to condense our findings in this volume.