Network analysis is often used to understand the behavior of groups of people. Relationships within a group of people form a kind of network—a social network. Social networks are some of the longest-studied in network science, and provide some of the results most directly applicable to everyday life. This chapter will introduce you to the elementary results in social network analysis.

Topics in this chapter include the following:

• Social networks: The history of social networks in network science
• Strong and weak ties: How to interpret and quantify the intensity of relationships
• The small world problem: Understanding how very large networks can be spanned by relatively short paths
• Contagion: How information, diseases, and anything else spreads over networks