In the past, treeshrews have commonly been considered basal members of the order Primates, or united with macroscelidids in the "insectivoran" clade Menotyphla. However, as a group they have no immediate living relatives and are best classified at ordinal rank (Butler, 1972, 1980; Dene et al., 1978; Luckett, 1980; McKenna and Bell, 1997). At a deeper phylogenetic level, scandentians apparently form a natural group with dermopterans and primates (Murphy et al., 2001b). Representatives of the order are confined to southern, eastern, and SE Asia both currently and in the fossil record, which extends back to the Middle Eocene in east Asia (McKenna and Bell, 1997). Most previous workers have arranged Scandentia as a monofamilial order, but recognition of two families (Tupaiidae and Ptilocercidae) more aptly conveys the anatomical disparity evident among the living treeshrews (see below).
Despite the attention paid to the higher-level phylogenetic relationships of treeshrews, a modern revision of species-level taxonomy in the group is still unavailable; the most recent comprehensive review remains that of Lyon (1913), a thorough but now long-outdated work. Chasen (1940), Ellerman and Morrison-Scott (1966), and Corbet (in Corbet and Hill, 1992) produced regional lists of named forms, but not critical systematic treatments, and the latter two listings are beset by overlumping. This account is likewise no substitute for a comprehensive systematic review of the order, but in its preparation I have examined all treeshrew specimens (including types) in the collections of the American Museum of Natural History, Field Museum of Natural History, Museum of Comparative Zoology, and National Museum of Natural History, as well as a number of type specimens stored in European collections. Starting points for further research are noted below.