Octocorals are found worldwide and are a significant part of the biodiversity in temperate marine biomes. Like most animals, corals have complex interactions with microbes, which likely have critical impacts on their health. However, very little is known about the structure and function of the octocoral microbiome or the interactions with their host. A new study examined the microbiome diversity and secondary metabolism of different octocoral species and healthy versus unhealthy tissue. Healthy octocorals have a microbiome that is distinct from the surrounding environment, host genus-specific, and defined by so-far uncultured groups of bacteria. In contrast, the microbiome of necrotic coral tissue shows large shifts in its community structure compared to healthy tissue and is colonized by seawater bacteria. Functional analysis showed that healthy corals were enriched in microbial genes associated with antiviral defense, host-symbiont recognition, micronutrient acquisition and heat-stress response. This was the first study to apply primer-less, metagenomic sequencing to octocorals, which allowed researchers to identify so-far uncultured, likely obligate symbionts as the core members of the octocoral microbiome. These results can be used to guide future studies into coral reef conservation and microbiome therapies for octocorals.