The presentation draws on long term ethnographic research in the Southern Peruvian Andes. The focus is on road construction and the knowledge practices of civil engineers, of the general workforce, and of local people. The materials from which roads are built are engaged in diverse ways: as matter, as persons, and as process. I am interested in the techniques through which people come to know these materials, in how particular evidential forms acquire social force and legitimacy, and in the diverse modes of expertise that have to accommodate to each other on construction sites.