Bay St. Louis is a city in and the county seat of Hancock County, Mississippi, in the United States. Located on the Gulf Coast on the west side of the Bay of St. Louis, it is part of the Gulfport–Biloxi Metropolitan Statistical Area. As of 2010, Bay St. Louis’ population was 9,260.

Biloxi (; French: [bilusi]) is a city in and one of two county seats of Harrison County, Mississippi, United States (the other seat being the adjoining city of Gulfport). The 2010 United States Census recorded the population as 44,054, and in 2019 the estimated population was 46,212. The area's first European settlers were French colonists. The city is part of the Gulfport–Biloxi metropolitan area and the Gulfport–Biloxi–Pascagoula, MS Combined Statistical Area. Prior to Hurricane Katrina, Biloxi was the third-largest city in Mississippi, behind Jackson and Gulfport. Due to the widespread destruction and flooding, many refugees left the city. Post-Katrina, the population of Biloxi decreased, and it became the fifth-largest city in the state, being surpassed by Hattiesburg and Southaven.The beachfront of Biloxi lies directly on the Mississippi Sound, with barrier islands scattered off the coast and into the Gulf of Mexico. Keesler Air Force Base lies within the city and is home to the 81st Training Wing and the 403d Wing of the U.S. Air Force Reserve.