This chapter introduces the idea that relationships are a fundamental component of human nature, but there is a limit on the number of relationship partners humans can have and maintain. This chapter introduces the idea of the core network (or the two to five most important people in your life), the first fifteen (i.e., the primary members of your personal network), and two outer layers of relationship closeness. This chapter explains why social interactions are a valuable unit of analysis for studying relationships, and why both personal relationships and social interactions are important contexts for the study of mobile and social media. This chapter introduces a relationship-centered approach to understanding media and presents assumptions about why relationships are important and how media is in the service of those relationships.