This paper deals with social capital as an extra-market externality, and its role for innovations and growth. It analyses the changes of innovation activity over time, from early industrialism to the global knowledge economy, how the relations between the actors of today's innovation systems have developed and the role of social networks for innovations. The different kinds of networks built by the three constructers of social networks: the individual, the organizations and the (public and civic) society are discussed. The role of public policy in building social capital for innovations and growth is analyzed.