A vehicle frame, also historically known as its chassis, is the main supporting structure of a motor vehicle to which all other components are attached, comparable to the skeleton of an organism.

Until the 1930s, virtually every car had a structural frame separate from its body, known as body-on-frame construction. Both mass production of completed vehicles by a manufacturer using this method, epitomized by the Ford Model T, and supply of rolling chassis to coachbuilders for both mass production (as by Fisher Body in the United States) and to smaller firms (such as Hooper) for bespoke bodies and interiors was practiced.

By the 1960s, unibody construction in passenger cars had become common, and the trend towards building unibody passenger cars continued over the ensuing decades.

Nearly all trucks and buses, and most pickups continue to use a separate frame as their chassis.

The main functions of a frame in a motor vehicle are:

Typically, the material used to construct vehicle chassis and frames include carbon steel for strength or aluminum alloys to achieve a more lightweight construction. In the case of a separate chassis, the frame is made up of structural elements called the rails or beams. These are ordinarily made of steel channel sections by folding, rolling, or pressing steel plate.

There are three main designs for these. If the material is folded twice, an open-ended cross-section, either C-shaped or hat-shaped (U-shaped), results. "Boxed" frames contain closed chassis rails, either by welding them up or by using premanufactured metal tubing.

By far the most common, the C-channel rail has been used on nearly every type of vehicle at one time or another.^{[citation needed]} It is made by taking a flat piece of steel (usually ranging in thickness from 1/8" to 3/16", but up to 1/2" or more in some heavy-duty trucks) and rolling both sides over to form a C-shaped beam running the length of the vehicle. C-channel is typically more flexible than (fully) boxed of the same gauge.