The advent of molecular techniques and their increasingly widespread use in public health laboratories and research studies has transformed the understanding of the burden of norovirus. Norovirus is the most common cause of community-acquired diarrheal disease across all ages, the most common cause of outbreaks of gastroenteritis, and the most common cause of foodborne disease in the United States. They are a diverse group of single-stranded RNA viruses that are highly infectious and stable in the environment; both symptomatic and asymptomatic infections are common. Through shedding in feces and vomit, norovirus can be transmitted directly through an array of routes: person-to-person, food or the environment. The relative importance of environmental transmission of virus is yet to be fully quantified but is likely to be substantial and is an important feature that complicates control.