Abstract Although the earliest arachnids were apparently marine, arachnid diversity has been dominated by terrestrial forms from at least the Devonian. Even though arachnid fossils are scarce (perhaps only 100 pre-Cenozoic taxa), representatives of all major arachnid clades are known or cladistically implied from the Devonian or earlier, suggesting very early origins (Selden and Dunlop 1998). The more recent great radiation of insects, in contrast, seems to be Permian (Kukalová-Peck 1991, Labandeira 1999). Taxonomically, arachnids today are composed of approximately 640 families, 9000 genera, and 93,000 described species (table 18.1), but untold hundreds of thousands of new mites and spiders, and several thousand species in the remaining orders, are still undescribed. Arachnida include 11 classically recognized recent clades, ranked as “orders,” although some acarologists regard Acari as a subclass with three superorders. Acari (ticks and mites) are by far the most diverse, with Araneae (spiders) second, and the remaining orders much less diverse. Discounting secondarily freshwater and marine mites, and a few semiaquatic spiders and one palpigrade, all extant arachnid taxa are terrestrial. Arachnids evidently arose in the marine habitat (Dunlop and Selden 1998, Selden and Dunlop 1998, Dunlop and Webster 1999), invaded land independently of other terrestrial arthropod groups such as myriapods, crustaceans, and hexapods (Labandeira 1999), and solved the problems of terrestrialization (skeleton, respiration, nitrogenous waste, locomotion, reproduction, etc.) in different ways.