Erie County is a county in the Commonwealth of Pennsylvania. It is the northernmost county in Pennsylvania. As of the 2020 census, the population was 270,876. Its county seat is Erie. The county was created in 1800 and later organized in 1803. The county is part of the Northwest Pennsylvania region of the state.

Warren County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 38,587. Its county seat is Warren. The county was established in 1800 from parts of Allegheny and Lycoming counties; attached to Crawford County until 1805 and then to Venango County until Warren was formally established in 1819.