Aggregate diffusion studies at the national level often depict the diffusion phenomenon as a one-to-one replacement process of “old” by “new”. In this essay it is argued that this can be highly misleading, as the case of mobility motorisation and the related displacement of the horse illustrate. Analysing three different instances of motorization (trains and tramways in the second half of the nineteenth century; urban truck and taxi fleets around the turn of the century, and agricultural motorisation), this essay shows that what is replaced are “functions” rather than artefacts. This becomes clear when the level of analysis is shifted from the national to the regional and local. Thus, seen from the artefact level, diffusion is a much more complicated process, fraught with asynchronicities, discontinuities and especially long-term coexistence of alternatives. It is the analysis of these phenomena which makes “diffusion” into a truly historical process.