Two counties in the state of Pennsylvania have populations of more than one million. These are Philadelphia County, which is coterminous with the city of Philadelphia and has a population of 1,574,765 (even though it is the second smallest county in the state by land area), and Allegheny County, with its 1,227,553 residents. As of 2018, both of these counties have grown their populations since the last census, with Philadelphia County increasing by 3.04% and Allegheny County by a more modest 0.30%.
A number of other counties in Pennsylvania, while not quite as large, still have substantial populations. Montgomery County (820,656), Bucks County (626,751), and Delaware County (563,995) are all in the top five most populous counties in the state, and census statistics show that each of these have also had population increases of between 0.21% and 2.46% in recent years. Conversely, many counties in Pennsylvania have populations of fewer than fifty thousand residents. These include Forest County (7,306), Juniata County (24,475), and Susquehanna County (41,238).
In general, the population increases in Pennsylvania's counties are fairly modest. None have increased by more than 6%, with Cumberland County topping the list at 5.09%, closely followed by Centre County at 5.06%. Quite a number of counties have had negative population growth: Cameron County, the least populous in the state, has just 4,683 residents and its population has decreased by 7.69% since the last census, while the larger Cambria County's population has reduced to 134,313, representing a 6.37% decrease.