An underground storage tank system (UST) is a tank and any underground piping connected to the tank that has at least 10 percent of its combined volume underground. The federal UST regulations apply only to UST systems storing either petroleum or certain hazardous substances.

When the UST program began, there were approximately 2.1 million regulated UST systems in the United States. Today there are far fewer since many substandard UST systems have been closed. For the most current statistics, see UST Performance Measures. Nearly all USTs regulated by the underground storage tank requirements contain petroleum. UST owners include marketers who sell gasoline to the public (such as service stations and convenience stores) and non-marketers who use tanks solely for their own needs (such as fleet service operators and local governments). EPA estimates that less than 10,000 tanks hold hazardous substances covered by the UST regulations.