In the 2009 swine flu outbreak, the source of the disease might have been a small village in Mexico, but the disease spread within months to all parts of the world. The use of air transportation by infected individuals led to a rapid diffusion of the disease. Also in 2009, negotiations on a climate treaty continued at the climate conference in Copenhagen. There was no global agreement on policies to address climate change, although international negotiations have been ongoing since the 1980s, alternative energy technology has become more accessible, and effects of climate change start have become measurable. The diffusion of norms and technology within this topic area is slow.

The study of diffusion focuses on the attributes of systems that affect the speed and scope of the diffusion. The social network structure can affect the spread of ideas and diseases, bits and bugs. When the average path length of agents within a network is small, the medium may spread faster. However, the diffusion speed and scope also depends on the medium of diffusion. Is an innovation or disease easy to acquire when one comes in contact with it? Does the transmission need physical contact, is it a sexual transmitted diseases, or is a cough sufficient for transmission? Can the idea spread via mass media, or does one need to train a person to acquire the skill?

In this chapter we discuss a number of examples of diffusion processes. We will look at the diffusion of innovations, the spread of diseases, the adoption of new products, and the dissemination of culture.