Downforce is a downwards thrust created by the aerodynamic characteristics of a car. The purpose of downforce is to allow a car to travel faster through a corner by increasing the vertical force on the tires, thus creating more grip.

The same principle that allows an airplane to rise off the ground by creating lift from its wings is used in reverse to apply force that presses the race car against the surface of the track. This effect is referred to as "aerodynamic grip" and is distinguished from "mechanical grip," which is a function of the car mass repartition, tires and suspension. The creation of downforce by passive devices almost always can only be achieved at the cost of increased aerodynamic drag (or friction), and the optimum setup is almost always a compromise between the two. The aerodynamic setup for a car can vary considerably between race tracks, depending on the length of the straights and the types of corners; some drivers also make different choices on setup. Because it is a function of the flow of air over and under the car, and because aerodynamic forces increase with the square of velocity, downforce increases with the square of the car's speed and requires a certain minimum speed in order to produce a significant effect. But some cars have had rather unstable aerodynamics, such that a minor change in angle of attack or height of the vehicle and this can cause large changes in the downforce. In the very worse cases this can cause the car to experience lift, not downforce, for example, caused by a bump on the track or slipstreaming over a crest, and sometimes can have disastrous consequences. A notorious example of this was Peter Dumbreck's Mercedes-Benz CLR in the 1999 Le Mans 24 hours, which flipped spectacularly after closely following a competitor car over a hump.