Decatur (dɪˈkeɪtə(r)) is a city in Morgan and Limestone counties in the U.S. state of Alabama. The city, nicknamed "The River City", is located in Northern Alabama on the banks of Wheeler Lake, along the Tennessee River. It is the largest city and county seat of Morgan County. The population in 2010 was 55,683.Decatur is also the core city of the two-county large Decatur, Alabama metropolitan area which had an estimated population of 153,374 in 2013. Combined with the Huntsville Metropolitan Area, the two create the Huntsville-Decatur Combined Statistical Area, of which Decatur is the second-largest city. Like many southern cities in the early 19th century, Decatur's early success was based upon its location along a river. Railroad routes and boating traffic pushed the city to the front of North Alabama's economic atmosphere. The city rapidly grew into a large economic center within the Tennessee Valley and was a hub for travelers and cargo between Nashville and Mobile, as well as Chattanooga and New Orleans. Throughout the 20th century, the city experienced steady growth, but was eclipsed as the regional economic center by the fast-growing Huntsville during the space race. The city now finds its economy heavily based on manufacturing, mining, cargo transit, chemical, and high-tech companies such as Vulcan Materials, Daikin, Toray, and United Launch Alliance.

Marshall County is a county of the state of Alabama, United States. As of the 2020 census the population was 97,612. Its county seat is Guntersville. A second courthouse is in Albertville. Its name is in honor of John Marshall, famous Chief Justice of the United States. Marshall County is a dry county, with the exception of the four cities of Albertville, Arab, Guntersville, and Boaz. Marshall County comprises the Albertville, AL Micropolitan Statistical Area, which is also included in the Huntsville-Decatur-Albertville, AL Combined Statistical Area.

Huntsville is a city in the Appalachian region of northern Alabama and is the county seat of Madison County. The city also extends west into neighboring Limestone County and south into Morgan County.It was founded in 1805 and became an incorporated town in 1811. The city grew across nearby hills north of the Tennessee River, adding textile mills, then munitions factories, NASA's Marshall Space Flight Center and the United States Army Aviation and Missile Command nearby at the Redstone Arsenal. The National Trust for Historic Preservation named Huntsville to its "America's Dozen Distinctive Destinations for 2010" list.The city's population is 215,006 as of the 2020 census, making it Alabama's most populous city. Huntsville is the largest city in the five-county Huntsville-Decatur-Albertville, AL Combined Statistical Area. The Huntsville metropolitan area's population was 492,000 as of 2020, making it the second most populous metropolitan area in the state after the Birmingham metropolitan area. The Huntsville metro's population reached 462,693 by 2018.

Athens is a city in and the county seat of Limestone County, in the U.S. state of Alabama; it is included in the Huntsville-Decatur-Albertville, AL Combined Statistical Area. As of the 2010 census, the population of the city is 21,897.