This text presents a key to understanding how ideas, products and opinions ""take off"" and spread throughout society - referred to as the diffusion of innovation - and provides a means to estimate how fast or slow that spread occurs. The diffusion of innovations occurs among individuals in a social system, and the pattern of communications among these individuals is a social network. The network determines how quickly innovations diffuse and the timing of each individual's adoption. The book thus analyses how social networks structure the diffusion of innovation. Chapter 1 reviews the diffusion of innovation theory, network analysis, and the datasets used as examples. Chapter 2 reviews prior research conducted on threshold and critical mass models. Chapter 3 presents relational network models of diffusion which posit that individuals adopt innovations based on their direct relations with others in their social system, and the next chapter provides structural network models of diffusion which posit that individuals adopt innovations based on their position in the social system, regardless of direct ties to specific others. Threshold and critical mass models of diffusion are presented in chapters 5 and 6. Chapter 7 develops a general threshold model of adoption based on social networks. This book is addressed to researchers, policymakers and students interested in diffusion of innovation or network analysis. It is also meant for those interested in studying the process of social change as represented by a diffusion network paradigm. Finally, individuals interested in developing, evaluating or understanding communication campaigns and media effects can use these concepts to improve their work.