Though initially beginning his research career in palaeontology with the intention of working on fossil marine reptiles, Naish became known for his doctoral work on the basal tyrannosauroid theropod "Eotyrannus", a dinosaur that he, together with Steve Hutt and colleagues, named in 2001. He has published articles on the Wealden Supergroup theropods "Thecocoelurus", "Calamospondylus" and "Aristosuchus". With Martill and Dino Frey, he named a new illegally acquired Brazilian compsognathid theropod "Mirischia". In 2004, Naish and Gareth Dyke reinterpreted the controversial Romanian fossil "Heptasteornis". Suggested by other authors to be a giant owl, troodontid or dromaeosaurid, it was argued by Naish and Dyke to be an alvarezsaurid, and as such is the first member of this group to be reported from Europe. Other fragmentary European alvarezsaurid specimens have since been reported.