Montgomery County (known colloquially as "Montco") is a county locate in the U.S. state of Pennsylvania, in the United States. As of 2000, the population was 750,097. A 2005 U.S. Census estimate placed the population at 795,618, making it the third most populous county in Pennsylvania (after Philadelphia and Allegheny counties), and the 69th most populous county in the United States. The county seat is Norristown.