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This paper analyses the diffusion of networks technologies as a complex and contingent process (as recommended in the call for papers for the New York City conference). Starting from the global process of change of the city, considered as a built environment, the first section of paper outlines three broad stages of transformation. A second section discusses the process of diffusion of several networks (water, electricity, telecommunications and automobile) with a focus on the involvement of different types of actors at the different stages of diffusion of a network: infancy, expansion, generalization. The third section addresses two major issues: i) the “splintering hypothesis,” examined in the light of the lessons of history and the logic of firms; and ii) the new challenge of regulating utilities, given that the universalization of networks gives rise to a mix where networks and practices, pipes and flux are intermingled.