The book summarizes what is known about MCEs geographically and by major taxa. Mesophotic coral ecosystems (MCEs) are characterized by light-dependent corals and associated communities typically found at depths ranging from 30-40 m and extending to over 150 m in tropical and subtropical ecosystems. They are populated with organisms typically associated with shallow coral reefs, such as corals, macroalgae, sponges, and fish, as well as species unique to mesophotic depths or deeper. During the past decade, there has been increasing scientific and management interest in MCEs. The questions covered include: How do MCEs vary geographically? What are the similarities and differences between MCEs and shallow reefs? Are shallow and mesophotic reefs connected? What are the conservation and management challenges and threats to mesophotic reefs.