Modified stock car racing, also known as modified racing and modified, is a type of auto racing that involves purpose-built cars simultaneously racing against each other on oval tracks. First established in the United States after World War II, this type of racing was early-on characterized by its participants' modification of passenger cars in pursuit of higher speeds, hence the name.

There are many sanctioning bodies for modifieds, each specifying different body styles and engine sizes.

A typical early "modified stock car" was, as its name implies, generally a stock automobile, with the glass removed, a roll cage installed, and a souped-up motor. NASCAR began by organizing the modifieds, and ran its first race in Daytona Beach in February 1948 at the beach road course. (In June 1949, NASCAR organized its first "strictly stock" later model car race at Charlotte, North Carolina, which evolved into its well known premiere division.)

What started out as minor modifications to the cars became visibly apparent as the bodies were channeled and lowered. The car builders started mixing and matching components from different car makers. Some modified classes are no longer based on any current production vehicles. Modified racing remained popular, particularly on the east coast, and grew away from "strictly stock" or "Late Models" and became akin to both stock cars and open-wheel cars. Until the early 1970s, drivers typically competed on both dirt and asphalt surfaces with the same car.

Modified cars resemble a hybrid of open wheel cars and stock cars. The rear wheels are covered by fenders but the front wheels and engine are left exposed. There are sanctioning bodies that control the rules for this class at most tracks. Each sanctioning body has their own set of guidelines provided in an annual rule book and their own registration fees.

Modifieds designed solely for asphalt surfaces began appearing in the early 1970s. These modifieds sit on large slick tires that are exposed on all four corners of the car. The roofs of these cars are more rounded than the other types of modifieds, their bodies look somewhat squashed and have large spoilers on the rear of the cars. Drivers sit on the left side of the car, and most of the time will have some type of small plastic windshield in front of them. Asphalt Modifieds are most popular in the Northeast but can be found from Florida to Texas to Michigan under different sanctioning bodies, as well as unsanctioned local classes. The rules packages may vary greatly.

The Midwest Modifieds Tour started in 1989 as the E-Mod Series at Mount Lawn Speedway near Indianapolis IN. It has undergone several name changes including ICAR, NSTA Top Speed Modified Tour, and the USA Modified Series. This travelling series races at local short tracks in Indiana, Michigan, and Ohio. In 2013 they raced for the first time on a road course, near South Haven, Michigan, at Gingerman Raceway. The series changed hands for 2017, from ICAR to NSTA, after being purchased by John Robbins (nephew of the former ASA pioneers) from Dave Muzillo.

The NASCAR Whelen Modified series is the only remaining NASCAR series from the sanctioning body's original season in 1948. The original style of NASCAR modifieds actually pre-dates NASCAR's existence by many years. Though it is now known as an asphalt-oval-only series, this was not always the case. The series originally started on dirt, and as more and more asphalt tracks opened, the series migrated to racing on the newer tracks. The series also had previously made forays into road racing with stops at Watkins Glen International in New York. Drivers like Bobby Allison and Red Farmer started in the NASCAR Modified series, which was popular both in the south and the Northeast. Many NASCAR Sprint Cup drivers have come from this series, including Ryan Preece, Jimmy Spencer, Geoff Bodine and Steve Park.