Book excerpt: 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.