Each year, Mattel releases a new assortment of Hot Wheels toys. The mainline (or mainline-up) consists of the standard 1/64 scale cars that are sold individually and are available at retail.  The mainline usually includes new castings (which typically debut under the First Editions series) as well as older castings that have been updated with new colours or tampographs for that year.

The mainline is distinct and separate from other toys which include playsets with pack-in cars, models produced at a scale other than 1/64, convention exclusive cars, and other special releases.