What kinds of relationships do people have when, due to rising divorce rates and declining birth rates, they have neither children nor relatives? Who do they turn to when family is no longer a safety net in old age? What form of social relationships do online contacts in social networks or workplace relationships represent in the modern working world? Friendship seems to be the obvious answer to these questions. This volume provides an introduction to the sociology of friendship for the first time. The contributions offer an overview of the sociological basics of friendship and insights into the diversity of friendship phenomena. Last but not least, the question is addressed: How does friendship manifest today? With guest contributions from Andrea Knecht, Christian Kühner, and Kai Marquardsen.