To what extent are our political views shaped by those with whom we associate? This chapter discusses the research on interpersonal networks as they relate to processes of attitude and opinion formation. Reviewing the literature’s origins, evolution, and essential design features, the chapter focuses largely on its development in and application to the US context. At base, much of the scholarship on core social networks in the mass public posits that (non-) exposure to disagreement influences what we think about politics and how we vote in elections. The chapter examines the array of findings surrounding this and other characteristics of our “offline” social circles and notes challenges to inference stemming from questions about causal identification, debates over measurement, and ambiguity concerning mechanisms. It closes by describing several potentially fruitful areas for future research and articulating a vision for a more unified approach to the study of persuasion in interpersonal networks.