The term ‘diffusion’ is widely used in many fields of science, where it refers essentially to the same phenomenon: the spread in space or acceptance in a human environment, over time, of some specific item or pattern. The phenomenon in question may be the spread of matter of a given type in physics, of an idea or pattern of culture in anthropology, of a practice or an institution in sociology, or of a product or a method of making one in economics. The primary interest of the (theoretical and empirical) diffusion studies in all these fields is equally common: to identify the factors that influence (facilitate or resist) the process of spread and then to discover the precise relation between these factors and the rate of spread within a given environment. There are also further, although more distant, similarities, some of which will become apparent in the course of this chapter. However, unlike physical phenomena, which do not need man to take place, anthropological or economic diffusion processes involve people in an essential way, as both diffusion agents and adopting units. This fact complicates the task of a social scientist enormously, since human abilities, values, and attitudes immediately come into play, and these are inevitably highly heterogeneous and essentially situation-specific.