Networks provide a unified way to think about the interaction between individuals or populations, and are especially useful when each individual is in direct contact with only a small proportion of the population. Networks tend to be very powerful tools for understanding the transmission of infection in human populations due to either social contacts (for airborne infections) or sexual contacts (for sexually transmitted diseases). In either case we expect that each individual will be in contact with only a small proportion of the population, and that the number of contacts will be highly heterogeneous—networks provide a simple means of capturing such interactions. We therefore see that the primary advantage of network models is their ability to capture complex individual-level structure in a simple framework.