Outbreaks of infectious disease are traditionally investigated using a combination of field epidemiology work - interviewing cases - and molecular epidemiology - laboratory techniques like genetic fingerprinting. Unfortunately, even the combined application of these two techniques is not enough to solve many outbreaks - the source of the outbreak may go undetermined, and the chain of transmission cannot be identified. Recently, however, the advent of next-generation DNA sequencing technologies has opened the door to a new area of molecular epidemiology - genomic epidemiology. By sequencing the whole genomes of all the isolates from an outbreak of infectious disease, epidemiologists can now use the presence or absence of individual mutations to track the person-to-person spread of disease. This talk will introduce the conceptual basis behind this new area and present some of the first outbreak reconstructions to result from the technique.