Trematode families and genera: have we found them all?

The proposal of new trematode families has almost stopped. Many new genera are still being proposed, but the number has fallen below historical rates. For most of the history of description of trematodes there have been more genera known from tetrapods than from fishes, but this pattern has reversed recently. These reductions are argued to be more of a reflection of the law of diminishing returns than diminution of effort. Thus, at the family level the classification of trematodes is becoming mature, and at the genus level we are seeing the 'beginning of the end' of the discovery of diversity. However, work for generations of scientists remains in other aspects of trematode biodiversity research, especially in life cycles, phylogeny and biogeography.