Colonel John Singleton Mosby led the most successful partisan campaign during the American Civil War. Major General J. E. B. Stuart, the Confederate Cavalry commander, allowed Mosby to form an unconventional unit with nine volunteers in December 1862. Mosby then organized, trained, and equipped the 43d Battalion of Virginia Cavalry, better known as Mosby's Rangers. This unit grew to almost one thousand men in strength and conducted many daring raids on the Union forces and lines of communication. Mosby, nicknamed the Gray Ghost, tied down Union troops in Virginia and Maryland and relieved pressure on General Robert E. Lee and the Army of Northern Virginia. The Rangers also provided timely and accurate intelligence to Confederate corps-level units. This study examines Mosby's campaign from the unit's formation in 1863 until the termination of the war in 1865. All aspects of this unconventional campaign are described with emphasis on Mosby's tactics, techniques, and procedures. This study provides an excellent example of the successful employment of partisan warfare and the attempts to combat it. It concludes with the lessons learned and their contribution to current Army doctrine.