Live hard coral cover is the primary indicator of the health of coral reefs, from local to larger scales, including national, regional and global. Hard coral cover is measured visually in situ or from photographic images or from mutispectral sensors, as the percentage cover of hard corals of the surface area of a coral community or assemblage. The indicator directly addresses the health of the ecosystem, and thus of management and policy measures taken to protect, maintain or restore the health of the coral reef. It answers questions such as ‘what is the status or health of a coral reef?’, or ‘what is the impact of a certain management or policy intervention?’.

The percent cover of hard corals has been measured in coral reef science and monitoring for over 100 years, recognized as a key variable for distinguishing a coral community from one dominated by other taxa (eg. algae, other invertebrates). It has been the primary measure for reef monitoring and reporting, in the scientific literature, and adopted by the Global Coral Reef Monitoring Network (GCRMN) (Wilkinson 2000). It is identified as one of the two most advanced Essential Ocean Variables in 2016 and as an ecosystem structure Essential Biodiversity Variable (Obura et al. 2019, Muller-Karger et al. 2018). The GCRMN Implementation and Governance Plan adopted in December 2018 defines it as the core variable for coral reef monitoring, supported by algal cover and fish diversity and biomass, for global monitoring and reporting of coral reef health.