Marine hard bottoms feature some of the most spectacular and diverse biological communities on this planet. These not only contain a rich treasure of genetic, taxonomic and functional information but also deliver irreplaceable ecosystem services. At the same time, they are highly vulnerable and increasingly threatened by anthropogenic pressures. This volume has collected contributions by 50 scientists from numerous biogeographic regions, dealing with characteristics of hard bottom communities. Distributional patterns in space and time are described, followed by analyses of the intrinsic and extrinsic dynamics producing these patterns. A strong emphasis is placed on the ongoing changes occurring in the structure and diversity of these communities in response to spiralling environmental impacts, and on state-of-the-art countermeasures aiming to preserve these ecological treasures. Finally, various values of diversity are assessed, hopefully as an incentive for enhanced conservation efforts.