Four types of diffusion processes exist. Relocation diffusion occurs when groups move from one area to another over time. Expansion occurs when new members join groups over time. Contagion diffusion occurs through interpersonal contact, and hierarchical diffusion occurs as information spreads through the levels of an organization. Elements in the diffusion process include the environment, time, the item being diffused, the point of origin, the destination, and paths of movement. An analysis of the diffusion of terrorist activity, using the ITERATE II statistical analysis system and adjacency maps which express the geographic relationships between countries, indicates that diffusion is hierarchical between regions at first and then becomes contagious within the regions. The analysis also shows that the rate of contagion varies according to region and is highest in Latin America, that border contiguity is a factor, that diffusion is particularly weak in subsaharan Africa, that 75 percent of the diffusion occurred during the first 5 years of the decade, and that high activity regions changed over time. The highest levels of activity were in Western Europe. Factors contributing to terrorism include general coordination among groups, governmental support, international terrorist conferences, ease of communications through the media, a lack of legal barriers, and general political and ideological affinity between groups. Terrorism has not spread easily where barriers existed to group formation or communication. Charts, footnotes, and maps are included. Related data are appended.