The article deals with the topic of social networks role in the development of the ICT firms. It is based on a piece of research carried out in four Italian local production systems: Torino, Firenze, Pisa and Osimo. These local economies have recently experienced a growth in the ICT productions: the first three cases in the "intangible services" sector (software houses, internet providers etc.) and the fourth one in the "manufacturing" sector (electronic components). The article explores the following issues: 1) the role of social networks in the origin of the ICT firms, underlining the differences with those operating in the manufacturing industrial districts, with specific reference to the use of familiar relational resources; 2) the importance of social networks in structuring the markets and the inter-firms cooperative agreements in the ICT industry; 3) the functions and the features of the entrepreneurs instrumental network and the different forms assumed by their social capital with regard to specific tasks; 4) the influence of the entrepreneurial network on the innovative and economic performances of the firms.