This work describes the corals (Anthozoa) and their structure - the coral reefs. Coral reefs are ecosystems of high biodiversity, which grow only in warm, shallow and clear seas. The key to this remarkable biological productivity and ability to excrete limestone to build coral reefs is a symbiotic relationship between corals and the photosynthetic algae Zooxanthellae. Corals allow Zooxanthellae preserved environment and supply them with compounds necessary for photosynthesis, while Zooxanthellae produce oxygen, help the coral to remove metabolic waste and supply them with organic products of photosynthesis. As corals grow and expand, reefs create one of the three main structures - the fringing reef, barrier or atoll. The most famous coral reefs, which are listed in this seminar are the Great Barrier Reef, the Mesoamerican Barrier Reef System, the New Caledonia Barrier Reef and the Paulley Ridge. Most of the reefs are on the UNESCO world heritage list, because they are faced with many dangers, both natural and anthropogenic.