Dinosaur diversity is analyzed in terms of the number of valid genera within each major clade, Mesozoic age, place of discovery and year of description. Aves (Archaeopteryx + Neornithes) is excluded. Nomina nuda and nomina dubia are not counted. The results show 451 valid dinosaurian genera at the end of 2001, of which 282 are saurischian (112 sauropodomorphs and 170 theropods, including 93 coelurosaurs) and 169 ornithischian, including 11 pachycephalosaurs, 26 ceratopsians, 60 ornithopods, 12 stegosaurs, and 38 ankylosaurs. Thirty-eight genera arose in the Triassic, 124 in the Jurassic, and 289 in the Cretaceous, of which a disproportionately high number — 85 and 47 — are from the Campanian and Maastrichtian. The Kimmeridgian was the most productive age, with an average of 11.18 new genera per million years. The Kimmeridgian saw an unparalleled boom in sauropod diversity, with 20 new sauropod genera arising in its 3.4 million years, an average of one new sauropod every 170,000 years. Asia was the most productive continent with 149 genera, followed by North America (135), Europe (66), South America (52), Africa (39), Australasia (9), and finally Antarctica (1). Just three countries account for more than half of all dinosaur diversity, with 231 genera between them: the U.S.A (105), China (73), and Mongolia (53). The top six countries also include Argentina (44), England (30), and Canada (30), and together provide 335 dinosaur genera, nearly three quarters of the total. The rate of naming new dinosaurs has increased hugely in recent years, with more genera named in the last 19 years than in all the preceding 159 years. The results of these analyses must be interpreted with care, as diversity in ancient ecosystems is perceived through a series of preservational and human filters yielding observed diversity patterns that may be very different from the actual diversity.