Marine sponges are functionally important components of benthic environments, however surprisingly little is known about the etiological origins of sponge disease. Sponge disease epidemics have had significant long‐term effects on some species, with the combined effects of commercial sponge harvesting and disease taking numerous sponge populations to the brink of extinction. Importantly, sponges are known to host stable and diverse communities of microbial symbionts which can account for up to 35% of total biomass and which likely carry out functions that support their host’s health and ecology, such as the transformation of carbon, nitrogen, and sulphur, as well as providing essential vitamins and amino acids to the host. Understanding whether disease symptoms are due to novel infectious agents or environmentally induced dysbiosis of the symbiotic microbiome has been the topic of considerable research. To date, only a single study has unequivocally assigned a collagenolytic pathogen as the causative agent of sponge disease, whilst many other studies have shown that symptoms of disease are concomitant with a shift in the composition of the native microbiome. Further work is required to understand the cause:effect pathway for disease onset, including the relatively unexplored role of viruses and protozoa in sponge disease. In parallel, studies should elucidate the role of the host immune response in disease development and identify environmental drivers of disease onset.