This article offers a critical analysis of the notions of causality and conditioning of linguistic change as these were developed in the French functionalist school, most notably by Martinet (Norsk tidsskrift for sprogvidenskap 18: 434–440, 1955), and some of his followers (Haudricourt, Juilland, Essai pour une histoire structurale du phonétisme français, Klincksieck, 1949, Hagège, Haudricourt, La phonologie panchronique, Presses Universitaires de France, 1978). In the first section, we analyse the types of causality discussed in functionalist diachronic studies. We then discuss the teleological aspect associated with functionalist causal explanations. Finally, we analyse the distinction between internal and external factors in linguistic change. These issues are inextricably intertwined in diachronic functionalism, but French functionalists adopt different stands with respect to the nature, role and function of internal and external factors, and with respect to the explanation of ‘actuated’ linguistic change. From our critical analysis it appears that the concept of causality has not been fully elaborated by the functionalists, and that the notions of causality and conditioning remain vague. We argue that this is due to a more basic problem at the level of the theoretical conception of language, viz. the fact that Martinet is caught between three perspectives on ‘language’: (1) the linguistic system as existing independently of speakers; (2) the individual speaker and the idiolect as a supposedly homogeneous object of study; (3) the socially stratified and heterogeneous speech community.