This chapter explores the concept and application of social network influence. It begins by defining node influence, discussing its theory and manifestations. Next, it focuses on measuring node influence based on network topology and examines how influence spreads within social networks. This chapter also covers evaluation metrics and practical uses of influence. Understanding social network influence aids in analyzing individual and group behavior, supports public decision-making and opinion analysis, and enhances security and development across social, cultural, and economic sectors. Thus, the study of social network influence holds significant theoretical and practical value.