We examine the emergence and uneven spatial distribution of depopulation across U.S. counties over the 1980-to-2010 period. Depopulation--sometimes referred to as "population implosion"--is distinguished by absolute population decline and is a function of the protracted out-migration, low and declining fertility, and population aging. Depopulation increasingly characterizes growth and decline processes in rural areas, both in the United States and Europe. It is symptomatic of fundamental changes in the local demographic structure (especially population aging) which eventually drains demographic resilience and, ultimately, the prospect of long term economic equilibrium and population growth. Our findings, based on data from the U.S. decennial Censuses and local-area data on fertility and mortality, suggest that chronic population decline has been both protracted and substantial in many parts of rural America. Indeed, nearly one-half of all nonmetropolitan counties reached their population peaks by 1950, in contrast to just 13 percent of metropolitan counties. Since 1990, over one-third of rural counties experienced depopulation; natural increase no longer offsets chronic out-migration of young people of reproductive age. Our study highlights important new findings and provides lessons about the underlying dynamics of depopulation in a period of low fertility and unprecedented population aging in the United States and other developed countries.