A trainer is a class of aircraft designed specifically to facilitate flight training of pilots and aircrews. The use of a dedicated trainer aircraft with additional safety features—such as tandem flight controls, forgiving flight characteristics and a simplified cockpit arrangement—allows pilots-in-training to safely advance their skills to a more unforgiving aircraft.

Civilian pilots are normally trained in a light aircraft, with two or more seats to allow for a student and instructor.

The two seating configurations for trainer aircraft are: pilot and instructor side by side, or in tandem, usually with the pilot in front and the instructor behind. The side-by-side seating configuration has the advantage that the pilot and instructor can see each other's actions, allowing the pilot to learn from the instructor and the instructor to correct the student pilot. The tandem configuration has the advantage of being closer to the normal working environment that a fast jet pilot is likely to encounter.

It is now the norm for pilots to begin their flight training in an aircraft with side-by-side seating and to progress to aircraft with tandem seating. This, however, has not always been the case. For example, it was usual to find tandem seating in biplane basic trainers such as the Tiger Moth and the Jungmann, and the British used side-by-side seating in the operational conversion of some of its fast jets such as the English Electric Lightning.

Given the expense of military pilot training, air forces typically conduct training in phases to eliminate unsuitable candidates. The cost to those air forces that do not follow a graduated training regimen is not just monetary but also in lives. For example, for many years, the Indian Air Force operated without a suitable advanced training aircraft, leading to a high casualty rate as pilots moved to high-performance MiG-21 aircraft without suitable assessment of their aptitude for supersonic flight.

There are two main areas for instruction: flight training and operational training. In flight training, a candidate seeks to develop their flying skills. In operational training, the candidate learns to use their flying skills through simulated combat, attack, and fighter techniques.

Typically, contemporary military pilots learn initial flying skills in a light aircraft not too dissimilar from civilian training aircraft. In this phase, pilot candidates are screened for mental and physical attributes. Aircraft used for this purpose include the Slingsby Firefly, as at one time used by the United States Air Force Academy, and the Scottish Aviation Bulldogs of the RAF. The U.S. replaced the Firefly and the Enhanced Flight Screen Program (EFSP) with the Diamond DA20 and the Initial Flight Training (IFT) program. At the end of this stage, pilot trainees are assessed, and those who pass advance to the full pilot training program. Those who are judged unsuitable for a pilot commission, but show other attributes, may be offered the chance to qualify as navigators and weapons officers. Smaller and more financially restricted air forces may use ultra-light aircraft, gliders, and motor gliders for this role. The USAF Academy uses light piston-powered aircraft such as the Cirrus SR20 (designated T-53A) for basic cadet flight training.

After the ab-initio phase, a candidate may progress to basic, or primary, trainers. These are usually turboprop trainers, like the Pilatus PC-9 and Embraer Tucano. Modern turboprop trainers can replicate the handling characteristics of jet aircraft as well as having sufficient performance to assess a candidate's technical ability at an aircraft's controls, reaction speed, and the ability to anticipate events. Prior to the availability of high-performance turboprops, basic training was conducted with jet aircraft such as the BAC Jet Provost, T-37 Tweet, and Fouga Magister. Those candidates who are not suitable to continue training as fast jet pilots may be offered flying commissions and be trained to fly multi-engined aircraft.