What are opinion leaders? How do they exert their influence? What are the consequences for opinion formation? What distinguishes personally known opinion leaders from online opinion leaders? Since its discovery in the 1940s, the concept of opinion leadership and the flow of communication has been the subject of social science research. This volume provides a clear overview of the central processes and phenomena of opinion leaders and opinion leadership from a communication science perspective. In addition to theoretical foundations and methodological approaches, the empirical findings of opinion leader research are summarized—from pioneering studies to the current discussion of opinion leaders in online and social media. The book serves as a textbook for students, a reference work for key studies and findings, and a starting point.