The original races were run on dirt tracks that got rutted and bumpy. The unmodified cars were not tough enough for this type of abuse, so NASCAR began allowing modifications to the stock cars to increase their durability. Over the years, more and more modifications were made, sometimes to increase safety see How NASCAR Safety Works for details and sometimes to improve competition. NASCAR strictly controls all of these modifications, which are spelled out in detail in the NASCAR rule book. Cars are checked for compliance with these rules at every race.

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