MCEs are found worldwide in tropical and subtropical waters. The existence of corals at mesophotic depths has been known since at least 1889, when Darwin reported the discovery of corals at depths of 128 m (Darwin 1889). However, it was not until the 1960s and 1970s that direct observation of MCEs began in earnest (Starck and Starck 1972, Wells 1973). As deep-diving technologies have advanced and been adopted by scientists, so has our ability to access and study MCEs. This chapter takes an in-depth look at some of the MCEs that have been studied to date (Figure 3.1) and demonstrates that while there are commonalities among MCEs, there are also differences; just as the shallow coral reefs of the Great Barrier Reef are similar but different from those found in the Florida Keys.