The diversity of marine fungi is reviewed based on new data on their occurrence on various substrata, unexplored habitats especially in the tropics, undescribed species, endophytic and cryptic taxa. Various estimates have been proposed for the number of marine fungi and this depends on whether the taxa are obligate, facultative or marine derived by isolation from various substrata or from seawater by culture methods. The current number of obligate marine fungi (537 species) is a gross underestimate. So where can further species be found? In this review, several fungal groups are identified that have been poorly surveyed for marine representatives and therefore a potential source of more marine species; these are marine derived fungi isolated from soils, sand, and water, planktonic fungi, deep-sea fungi, unculturable fungi, and cryptic species or taxa with similar morphology. In accounting for these potential sources the number of marine fungi might be in excess of 10,000 taxa.