Coastline counties of the United States, located along the country’s saltwater edges, account for just 254 of the nation’s 3,142 counties yet contain 29 percent of its population, 5 of its 10 most populous cities, and 7 of its 10 most populous counties. Bordering the Atlantic and Pacific Oceans and the Gulf of Mexico, these bands of counties provide the setting for an intense concentration of economic and social activity. Shaped by migration, trade, and environmental factors, population trends in coastline counties are distinct from those for the nation as a whole or its noncoastline counties.