In this chapter, we provide an overview of the epidemiological literature on the association between driver distraction and the risk of crash. We examine the types of distracting activities that have been studied to date, summarize the results, and discuss future directions. The chapter begins with a defi nition of risk, an explanation of the hierarchy of epidemiological evidence, and a discussion about the types of study designs used by epidemiologists to identify whether driver distraction is a risk factor for having a crash.