The modern day is often caricatured by vertebrate biologists and paleontologists as the ”˜Age of Mammals,’ remote from the ”˜Age of Fishes’ of the Devonian Period more than 350 million years ago. This reinforces a common perception that living fishes, which collectively constitute half of all living backboned animals, have exceptionally deep evolutionary roots. However, a substantial fraction of diversity in this other half of the vertebrate tree of life was generated on a timescale comparable to that in birds and mammals. Drawing on the clues locked in genomes, skeletons, and rocks, I will explore the origins of diversity in fishes at phylogenetic scales ranging from individual specialized lineages to hyperdiverse radiations containing several thousand species. I will show how quantitative approaches to evolutionary questions in ”˜deep time’ highlight potential effects of major events in earth history on the assembly of the modern fauna, and illustrate how cutting-edge techniques are providing us with an unprecedented view of diversity deep within the geological record.