A colonel in the United States Army, Marine Corps, Air Force and Space Force, is the most senior field-grade military officer rank, immediately above the rank of lieutenant colonel and just below the rank of brigadier general. Colonel is equivalent to the naval rank of captain in the other uniformed services. By law, an officer previously required at least 22 years of cumulative service and a minimum of three years as a lieutenant colonel before being promoted to colonel. With the signing of the National Defense Authorization Act of 2019, military services now have the authorization to directly commission new officers up to the rank of colonel. The pay grade for colonel is O-6.

The Regular Army of the United States succeeded the Continental Army as the country's permanent, professional land-based military force. In modern times, the professional core of the United States Army continues to be called the Regular Army. From the time of the American Revolution until after the Spanish–American War, state militias and volunteer regiments organized by the states supported the smaller Regular Army of the United States. These volunteer regiments came to be called United States Volunteers (USV) in contrast to the Regular United States Army (USA). During the American Civil War, about 97 percent of the Union Army was United States Volunteers.