Following the exciting exploration of hot vent and cold seep ecosystems, the rediscovery of cold-water coral ecosystems with high-technology instrumentation is currently another hot topic in multidisciplinary marine research. Conventionally, coral reefs are regarded as restricted to warm and well-illuminated tropical seas, not associated with cold and dark waters of higher latitudes. However, ongoing scientific missions have shed light on the global significance of this overlooked ecosystem. Cold-water coral ecosystems are involved in the formation of large seabed structures such as reefs and giant carbonate mounds, and they represent unexploited paleo-environmental archives of earth history. Like their tropical cousins, cold-water coral ecosystems harbour rich species diversity. Despite the great water depths, commercial interests overlap more and more with the coral occurrences. Human activities already impinge directly on cold-water coral reefs causing severe damage to this vulnerable ecosystem. In this volume, the current key institutions involved in cold-water coral research have contributed 62 state-of-the-art articles from geology and oceanography to biology and conservation.