This article makes two fundamental contributions to evolutionary theories of technological changes. First, a socio-cognitive evolutionary perspective is developed that conceptualises the emergence of new technologies in the pre-market phase. This topic is underdeveloped in evolutionary economics, because of its emphasis on market selection. Second, the article addresses co-evolution between two competing technical trajectories. In the pre-market phase, this interaction occurs via expectations and social networks. The perspective is illustrated with a longitudinal case study of biogas plants in Denmark. This sustainability innovation exists in two forms: farm-scale plants and centralised plants. Both forms are carried by different populations and based on different cognitive rules.