Incidence is a measure of disease occurrence in a population at risk over a specified period of time. There are three key parts to this definition: first, incidence measures only new events of interest (e.g., a new case of disease); second, the new cases are measured only within a population that is considered to be at risk of the event occurring (e.g., women for cervical cancer); and third, there is always a specified amount of time. We will look at specific examples as we discuss the two different measures of incidence.