Social networks representing the pattern of social interactions - who talks to or who observes whom- play a crucial role as a medium for the spread of information, ideas, diseases, products. Someone in the population may be struck with an infection or may adopt a new technology, and it can then either die out quickly or spread throughout the population, depending pos- sibly on the location of the initial appearance, the structure of the network - for instance, how dense it is. The dynamics of adoption -the extent to which individuals are in uenced by their neighbours, the impact of \word- of- mouth" communication- also plays a role in determining the speed of di usion. Given the large range of contexts in which social learning is im- portant, it is not surprising that researchers from various disciplines have studied processes of di usion from a variety of perspectives.