Fungi play a dominant role in terrestrial environments where they thrive in symbiotic associations with plants and animals and are integral to nutrient cycling in diverse ecosystems. Everywhere that moisture and a carbon source coexist in the terrestrial biosphere, fungi are expected to occur. We know that fungi can be devastating to agricultural crops, both in the field and during their storage, and cause mortality in immunocompromised patients in numbers that rival the deaths from malaria. Yet fungi can also be harnessed as sources of food, chemicals and biofuels when humans exploit fungal metabolism. Despite their central role in the health and disease of the terrestrial biosphere, much less is known about the function and potential of marine fungi. Are fungi ubiquitous in marine environments as they are on land? Do they play the same or similar roles in these ecosystems? Here we describe the state of knowledge about the abundance and functions of fungi in the marine environment with a goal to stimulate new inquiry in this very open area.