Slats are aerodynamic surfaces on the leading edge of the wing of a fixed-wing aircraft which, when deployed, allow the wing to operate at a higher angle of attack. A higher coefficient of lift is produced as a result of angle of attack and speed, so by deploying slats an aircraft can fly at slower speeds, or take off and land in shorter distances. They are used during takeoff and landing or while performing low speed maneuvers which may take the aircraft close to a stall. Slats are retracted in normal flight to minimize drag.
Slats are high-lift devices used on high-speed turbojet aircraft. Trailing-edge flap systems running along the trailing edge of the wing are common on all aircraft.
The chord of the slat is typically only a few percent of the wing chord. The slats may extend over the outer third of the wing, or they may cover the entire leading edge. Many early aerodynamicists, including Ludwig Prandtl, believed that slats work by inducing a high energy stream to the flow of the main airfoil, thus re-energizing its boundary layer and delaying stall.  In reality, the slat does not give the air in the slot a high velocity (it actually reduces its velocity) and also it cannot be called high-energy air since all the air outside the actual boundary layers has the same total heat. The actual effects of the slat are:  
The slat has a counterpart found in the wings of some birds, the alula, a feather or group of feathers which the bird can extend under control of its "thumb".
Slats were first developed by Gustav Lachmann in 1918. The stall-related crash in August 1917 of a Rumpler C aeroplane prompted Lachmann to develop the idea, and a small wooden model was built in 1917 in Cologne. In Germany in 1918 Lachmann presented a patent for leading-edge slats.  However, the German patent office at first rejected it, as the office did not believe the possibility of postponing the stall by dividing the wing.
Independently of Lachmann, Handley Page Ltd in Great Britain also developed the slotted wing as a way to postpone the stall by delaying separation of the flow from the upper surface of the wing at high angles of attack, and applied for a patent in 1919; to avoid a patent challenge, they reached an ownership agreement with Lachmann. That year, an Airco DH.9 was fitted with slats and test flown.  Later, an Airco DH.9A was modified as a monoplane with a large wing fitted with full-span leading edge slats and trailing-edge ailerons (i.e. what would later be called trailing-edge flaps) that could be deployed in conjunction with the leading-edge slats to test improved low-speed performance. This was later known as the Handley Page H.P.20  Several years later, having subsequently taken employment at the Handley-Page aircraft company, Lachmann was responsible for a number of aircraft designs, including the Handley Page Hampden.
Licensing the design became one of the company's major sources of income in the 1920s. The original designs were in the form of a fixed slot near the leading edge of the wing, a design that was used on a number of STOL aircraft.
During World War II, German aircraft commonly fitted a more advanced version of the slat that reduced drag by being pushed back flush against the leading edge of the wing by air pressure, popping out when the angle of attack increased to a critical angle. Notable slats of that time belonged to the German Fieseler Fi 156 Storch. These were similar in design to retractable slats, but were fixed and non-retractable. This design feature allowed the aircraft to takeoff into a light wind in less than 45 m (150 ft), and land in 18 m (60 ft). Aircraft designed by the Messerschmitt company employed automatic, spring-loaded leading-edge slats as a general rule, except for the Alexander Lippisch-designed Messerschmitt Me 163B Komet rocket fighter, which instead used fixed slots built integrally with, and just behind, the wing panel's outer leading edges.
Post-World War II, slats have also been used on larger aircraft and generally operated by hydraulics or electricity.
Several technology research and development efforts exist to integrate the functions of flight control systems such as ailerons, elevators, elevons, flaps, and flaperons into wings to perform the aerodynamic purpose with the advantages of less: mass, cost, drag, inertia (for faster, stronger control response), complexity (mechanically simpler, fewer moving parts or surfaces, less maintenance), and radar cross-section for stealth. These may be used in many unmanned aerial vehicles (UAVs) and 6th generation fighter aircraft.
One promising approach that could rival slats are flexible wings. In flexible wings, much or all of a wing surface can change shape in flight to deflect air flow. The X-53 Active Aeroelastic Wing is a NASA effort. The adaptive compliant wing is a military and commercial effort.   
A fluid flowing around an object exerts a force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the force parallel to the flow direction. Lift conventionally acts in an upward direction in order to counter the force of gravity, but it can act in any direction at right angles to the flow.
A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expressed as its lift-to-drag ratio. The lift a wing generates at a given speed and angle of attack can be one to two orders of magnitude greater than the total drag on the wing. A high lift-to-drag ratio requires a significantly smaller thrust to propel the wings through the air at sufficient lift.
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15°, but it may vary significantly depending on the fluid, foil, and Reynolds number.
A leading-edge extension (LEX) is a small extension to an aircraft wing surface, forward of the leading edge. The primary reason for adding an extension is to improve the airflow at high angles of attack and low airspeeds, to improve handling and delay the stall. A dog tooth can also improve airflow and reduce drag at higher speeds.
A swept wing is a wing that angles either backward or occasionally forward from its root rather than in a straight sideways direction.
An airfoil or aerofoil is a streamlined body that is capable of generating significantly more lift than drag. Wings, sails and propeller blades are examples of airfoils. Foils of similar function designed with water as the working fluid are called hydrofoils.
Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude.
Blown flaps, or jet flaps, are powered aerodynamic high-lift devices used on the wings of certain aircraft to improve their low-speed flight characteristics. They use air blown through nozzles to shape the airflow over the rear edge of the wing, directing the flow downward to increase the lift coefficient. There are a variety of methods to achieve this airflow, most of which use jet exhaust or high-pressure air bled off of a jet engine's compressor and then redirected to follow the line of trailing-edge flaps.
In aircraft design and aerospace engineering, a high-lift device is a component or mechanism on an aircraft's wing that increases the amount of lift produced by the wing. The device may be a fixed component, or a movable mechanism which is deployed when required. Common movable high-lift devices include wing flaps and slats. Fixed devices include leading-edge slots, leading edge root extensions, and boundary layer control systems.
A flap is a high-lift device used to reduce the stalling speed of an aircraft wing at a given weight. Flaps are usually mounted on the wing trailing edges of a fixed-wing aircraft. Flaps are used to reduce the take-off distance and the landing distance. Flaps also cause an increase in drag so they are retracted when not needed.
A leading-edge cuff is a fixed aerodynamic wing device employed on fixed-wing aircraft to improve the stall and spin characteristics. Cuffs may be either factory-designed or an after-market add-on modification.
A leading-edge slot is a fixed aerodynamic feature of the wing of some aircraft to reduce the stall speed and promote good low-speed handling qualities. A leading-edge slot is a spanwise gap in each wing, allowing air to flow from below the wing to its upper surface. In this manner they allow flight at higher angles of attack and thus reduce the stall speed.
A foil is a solid object with a shape such that when placed in a moving fluid at a suitable angle of attack the lift is substantially larger than the drag. If the fluid is a gas, the foil is called an airfoil or aerofoil, and if the fluid is water the foil is called a hydrofoil.
Boundary layer control refers to methods of controlling the behaviour of fluid flow boundary layers.
A circulation control wing (CCW) is a form of high-lift device for use on the main wing of an aircraft to increase the maximum lift coefficient. CCW technology has been in the research and development phase for over sixty years. Blown flaps were an early example of CCW.
The wing configuration of a fixed-wing aircraft is its arrangement of lifting and related surfaces.
The Handley Page H.P.20 was an experimental monoplane modification of a de Havilland DH.9A, built to study controllable slots and slotted ailerons as high lift devices. It was the first aircraft to fly with controllable slots.
A supersonic airfoil is a cross-section geometry designed to generate lift efficiently at supersonic speeds. The need for such a design arises when an aircraft is required to operate consistently in the supersonic flight regime.
The leading-edge droop flap is a device on the leading edge of aircraft wings designed to improve airflow at high pitch angles. The droop flap is similar to the leading-edge slat and the Krueger flap, but with the difference that the entire leading edge section rotates downwards, whereas the slat and Krueger flap are panels which move away from the wing leading edge when it is deployed.
The Villiers XXIV or Villiers 24 CAN2 was a French army night fighter most notable as the first French military aircraft to be fitted with leading edge slats.