A Drill Instructor (DI), Drill Sergeant (DS), or Military Training Instructor (MTI) is a non-commissioned officer in the military who is assigned the duty of initiating new recruits entering the military into the customs and practices of military life.

The drill instructor's responsibilities usually include instilling military discipline, improving recruits' physical fitness, and initial weapons training.

Drill Instructors in the United States Marine Corps normally hold the rank of sergeant through gunnery sergeant, while Drill Sergeants in the United States Army are staff sergeant or sergeant first class . Military Training Instructors in the United States Air Force are generally staff sergeants through master sergeants. They are held responsible for the welfare, behavior and military education of the recruits assigned to them on a twenty-four-hour basis throughout the period of initial training, popularly known as boot camp.

In the U.S. Army, soldiers (of appropriate rank) may be selected or volunteer to go to Drill Sergeant School. The school is nine weeks long and consists of the exact same activities as basic training (drill and ceremony, basic rifle marksmanship, obstacle/confidence courses, field training exercises). The prospective drill sergeants are treated just like new recruits. In some cases a student may find himself being yelled at and ordered around by a soldier of lesser rank. This is quite a change from the respect normally paid to a staff sergeant or platoon sergeant.

A U.S. Army drill sergeant's normal tour of duty is two years (with a possible one year extension), during which time he will train approximately eleven cycles of nine weeks each. The breaks between cycles are extremely short: a cycle will usually graduate on a Thursday or Friday, and new recruits will arrive the following Monday or Tuesday. Breaks between Marine Corps cycles are even shorter: after a class graduates the thirteen week cycle on Friday, a new class is picked up the following morning.

Statistics show that most cases of trainee abuse occur in the first six months and last six months of drill sergeant duty. Drill sergeants refer to these periods as 'the red zone'.

Because of the extremely arduous and demanding nature of this duty, drill instructor assignments are regarded as among the most prestigious carried out by enlisted military personnel. Those who become drill instructors are eligible for a variety of military awards, such as the Drill Instructor Ribbon and Drill Sergeant Identification Badge.