AbstractThe Arthropda are a monolphyletic taxon sharing many synapomorphies. The alternative models of polyphyletic origins of the arthropods are shown to be the result of 1. wrong interpretation of data, e. g. interpretation of the tracheate mandible as a “whole limb mandible”, 2. typological thinking, defining taxa on the basis of their overall similarity, and 3. unscientific reasoning, postulation unkown palaeozoic taxa instead of known ones as closest relatives.The main controversy concerns the position of the Tracheata. Are they Mandibulata and the sister group of the Crustacea or are they “Uniramia” and the sister group of the Onychophora? Whereas the Tracheata and Onychophora share only symplesiomorphies, the Tracheata share convincing synapomorphies with the Crustacea. The most important of these are the transformation of the third head appendage to the principal part of the mouth apparatus – this would even be true if the tracheate mandible were a “whole lim mandible”–, and the identical composition of the compound eye ommatidia.The Chelicerata are used as an example, following Lauterbach'S considerations, to demonstrate how fossils enhance our understanling of the evolutionary histor