We are surrounded by networks. The Internet, one of the most advanced artifacts ever created by mankind, is the paradigmatic example of a "network of networks" with unprecedented technological, economical and social ramifications. Online social networks have become a major driving phenomenon on the web since the Internet has expanded as to include users and their social systems in its description and operation. Technological networks such as the cellular phone network or the energy grid support many aspects of our daily life. Moreover, there is a growing number of highly-popular user-centric applications in Internet that rely on social networks for mining and filtering information, for providing recommendations, as well as for ranking of documents and services. Successfully striking examples are collaborative recommendation systems (e.g., Amazon for books and Netflix for DVDs), folksonomies - systems of collaborative social tagging - (e.g., Citeulike, Delicious, Flickr and Youtube), cooperative systems for building repositories of information as Wikipedia, systems of social networking (e.g., myspace, Facebook) and forum of discussion and opinion formation (e.g., blogs and Web communities). In this course we will present the design principles and the main structural properties and theoretical models of on-line social networks and technological networks, algorithms for data mining in social networks, and a few network economic issues, with an eye towards the current research issues in the area.