The term shoe-leather epidemiology is often synonymous with field epidemiology or intervention epidemiology. All 3 terms imply investigations initiated in response to urgent public health problems and for which the investigative team does much of its work in the field (i.e., outside the office or laboratory). Alexander D. Langmuir is credited with articulating the concept of disease surveillance as it is applied to populations rather than individuals. He also founded the Epidemic Intelligence Service (EIS) Program in 1951, a 2-year training experience in applied epidemiology that places professionals in the field, domestically and internationally, in real-life situations. Today, 70-90 EIS officers are assigned each year to Centers for Disease Control and Prevention programs and to state and local health departments to meet the broad spectrum of challenges in chronic disease, injury prevention, violence, environmental health, occupational safety and health, and maternal and child health, as well as infectious diseases. Throughout their assignments, EIS officers are encouraged to strive for analytic rigor as well as public health consequence, which requires technical competence blended with good judgment and awareness of context. Effective applied epidemiologists must have skills beyond just epidemiology to improve a population's health; the field of applied epidemiology requires multiple team members, all having different but complementary skills, to be effective.