Lehigh County is a county located in the Lehigh Valley region of the eastern part of the U.S. state of Pennsylvania. As of the 2010 census, the population was 349,497. Its county seat is Allentown, the state's third largest city behind Philadelphia and Pittsburgh. In addition to Allentown, the county includes the western section of the city of Bethlehem, six boroughs and 14 townships. Of 67 Pennsylvania counties, it is the 11th most populous. It is part of the New York City Metropolitan Area.
It is currently the fastest growing county in the entire state of Pennsylvania, and amongst the fastest growing nationwide from 2010 to 2012, ranking in the 79th percentile overall for that time period.
The county, which was first settled around 1730, was formed in 1812 with the division of Northampton County into two counties. It is named after the Lehigh River, whose name is derived from the Delaware Indian term Lechauweki or Lechauwekink, meaning "where there are forks".