Features that characterize many adapids include small orbits (eye sockets), elongate rostra, cheek teeth adapted for folivorous or frugivorous diets, and relatively large body mass (i.e., greater than 1 kg). However, the endemic radiation of adapids in the early and middle Eocene of Europe included a number of taxa (e.g., Anchomomys) that were very small (about 250 g or less) and partly insectivorous. Small orbits in genera such as Notharctus, Smilodectes, Adapis, Leptadapis, and Mahgarita indicate that these taxa were probably diurnal. At least one adapid genus from the late Eocene of Europe (Pronycticebus) had large orbits and was probably nocturnal.
Like living primates, adapids had grasping hands and feet with digits tipped by nails instead of claws. Other aspects of the postcranium suggest that most adapids were arboreal quadrupeds. North American endemic adapids (notharctines) like Notharctus had extremely long digits and postcranial proportions that superficially resemble those of living lemurid primates. By contrast, some authors have suggested that one group of late Eocene European adapids (adapines) demonstrate adaptations for suspension and slow climbing.
Adapid systematics and evolutionary relationships are controversial, but there is fairly good evidence from the postcranium that adapids are stem strepsirrhines (early members of the group including the living lemurs, lorises, and bushbabies). In particular, the anatomy of the adapid ankle (e.g., position of the groove for the flexor fibularis tendon on the talus, the presence of a sloping talo-fibular facet) and wrist show derived similarities with those of living strepsirrhines. However, adapids also lack many of the anatomical specializations characteristic of living strepsirrhines, such as a tooth comb, a grooming claw on the second pedal digit, and a reduction in the size of the promontory branch of the internal carotid artery.