The conclusion reached in this paper is that the cynodonts evolved from a therocephalian ancestor, and that among the known therocephalians, the whaitsiids are the forms closest to cynodont ancestry. Certain superficial specializations of the feeding apparatus, however, debar the known whaitsiids from a position of actual cynodont ancestry. The evidence for this thesis lies in new detailed morphology of certain points of the whaitsiid skull, along with reassessment of much of the established cranial anatomy of the relevant groups. It is presented in the form of a series of comparisons of the skulls of whaitsiids, primitive cynodonts, other therocephalians and primitive therapsids respectively, and a summary of this is given in table 1. The morphology of the whaitsiid skull is then discussed from a functional point of view, with particular reference to the design of bones as adaptations to resist the forces involved in the use of the jaws. The changes which must have occurred in the evolution of the cynodonts from a whaitsiid-like therocephalian are considered in this same context and it is argued that the organization of the cynodont skull can be seen as a logical functional development from the more primitive condition. In particular, the streptostylic nature of the jaw articulation, the enlargement of the dentary and reduction of the postdentary bones, and the reduction of the reflected lamina of the angular may all be correlated with the development of a masseter muscle.