Modified racing began as race drivers modified their cars to gain a competitive advantage over their fellow competitors. What started out as minor modifications to the cars has now grown to the point that some modified classes are no longer based on any current production vehicles. NASCAR was instrumental in modified racing's beginnings. Some of NASCAR's pioneers were veteran modified racers long before NASCAR's inception in 1949. As time progressed, modifieds grew away from "Strictly Stock" or "Stock Cars," and became akin to both stock cars, and open-wheel cars. Today's SuperModifieds have more in common with Indy cars than they do stock cars.

These modifieds are found mostly in the Eastern half of the US. Both of the sanctioning bodies (NASCAR and ASA) listed here use the same rules, but do not race on the same tracks. The cars are easy to identify. 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 listed in this article, their bodies look somewhat squashed, and have large, 8-inch-high (200 mm), Lexan spoilers on the rear of the cars. The driver sits on the left side of the car, and most of the time will have some type of small plastic windshield in front of him/her. All of these cars will sport small NASCAR or ASA stickers on the doors.

The largest builder of modifieds in the U.S. is former driver Maynard Troyer, who (after retiring in 1982) set up shop and in the 1980s was turning out 100 asphalt modifieds annually.

The NASCAR Whelen Modified series is the only remaining NASCAR series remaining 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. Many NASCAR Sprint Cup drivers have come from this series, including Jimmy Spencer, Geoff Bodine and Steve Park.

In late 2004, NASCAR bought out the Southern Modified Auto Racing Teams (SMART), and promptly re-named it the NASCAR Whelen Southern Modified Tour. The Southern Tour uses the exact same set of rules as the Northern Tour, but races primarily in the southeastern section of the US. Once per year, the Northern tour and the Southern tour race at Martinsville Speedway, near Martinsville, Virginia.

NASCAR's Whelen Southern Modified Tour has a direct competitor in the ASA Southern Modified Tour. The ASA Southern Modified Tour came about as some tracks that were part of the NASCAR buyout of SMART were unable, or unwilling to join NASCAR. The American Speed Association (ASA) was contacted, and the new series was formed. Although both sanctioning bodies use the same rules (much as NASCAR and ARCA did with the Sprint Cup series and the ARCA Re/Max Series), they do not share a single race date, nor do they race at any of the same tracks.

Dirt modifieds are probably the most popular type of racing in the US. Almost every dirt sanctioning body has a class of modifieds. Identifying these cars gets a little tricky at times.

The Super DIRTcar Series modifieds are the easiest to identify. The driver sits in the middle of the car, with high-downforce, wind-channeling tunnels on either side of him. The roofs are very flat, and tilted to catch additional air. The front suspension is usually a coil-over setup, with a torsion-bar set-up for the rear suspension. They utilize full tube chassis, which to the untrained eye, looks to be a sprint car chassis, but is much different in reality.

The IMCA-style modifieds are easy to identify as a whole, but distinguishing between the sanctioning bodies is all but impossible. The bodies are very flat on the sides, and lack the downforce generating tunnels the DIRTcar modifies sport. The driver sits on the left side. One of the most notable differences between the IMCA-style modifieds and other modified series cars, is the use of stock production car frame sections as part of the racing chassis. These cars also race on smaller tires than the other types of modifieds, with most sanctioning bodies specifying the same tire. The IMCA Sportmods appear very similar to their brethren, but have distinctly different engines. The Afco KidModz appear the same as the others, but sport full tube chassis, and by rules, have to have Ford 2300 cc inline 4 cylinder engines.

A modern Super DIRTcar Series modified in action.