This chapter explores how information, diseases, and innovations spread through social networks. It defines diffusion as a complex, network-based process influenced by social structures, node attributes, and dissemination mechanisms. Models such as independent cascade, linear threshold, and disease models like SIS and SIR are introduced. This chapter highlights the role of platforms like microblogging network, where features like passive forwarding accelerate the speed and reach of information. Different diffusion forms—interpersonal, group, organizational, and mass diffusion—are discussed, showing how they shape access to social and cultural capital in digital societies.