Although crowded housing is often cited as undesirable, the reasons why this condition is undesirable, the standards to use to measure crowding, and the point at which crowding becomes a housing defect are not well understood. This report concentrates on the effect that crowded housing has on its occupants and does not consider its possible effects on neighbors or neighborhoods. The report reviews the literature on crowding and reports the findings of previous studies that examined the standards used and the undesirable outcomes associated with crowding—chiefly in the areas of health and accidents. Using data from the 1985 and 2005 American Housing Survey, the study evaluates the incidence of crowding in the United States using a various measures: persons per room, persons per bedroom, square footage per person, and a hybrid that combines people per room and square footage per person. Although the incidence of crowding is low and has declined between 1985 and 2005, certain subgroups (such as foreign-born noncitizens) continue to experience relatively high levels of crowding.