Penobscot County is a county in the U.S. state of Maine, named for the Penobscot people in Wabanakik. As of the 2020 census, the population was 152,199, making it the third-most-populous county in Maine. Its county seat is Bangor. The county was established on February 15, 1816, from part of Hancock County when the area was still part of Massachusetts. Penobscot County is home to the University of Maine.

Waldo County is a county in the state of Maine, United States. As of the 2020 census, the population was 39,607. Its county seat is Belfast. The county was founded on February 7, 1827, from a portion of Hancock County and named after Brigadier-General Samuel Waldo, proprietor of the Waldo Patent.