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CERCLA hazardous substances are substances that are considered severely harmful to human health and the environment. Many are commonly used substances which are harmless in their normal uses, but are quite dangerous when released. They are defined in terms of those substances either specifically designated as hazardous under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), commonly known as the Superfund law, or those substances identified under other laws. In all, the Superfund law designates more than 800 substances as hazardous, and identifes many more as potentially hazardous due to their characteristics and the circumstances of their release.

Superfund's definition of a hazardous substance includes the following:

• Any element, compound, mixture, solution, or substance designated as hazardous under section 102 of CERCLA.
• Any hazardous substance designated under section 311(b)(2)(a) of the Clean Water Act (CWA), or any toxic pollutant listed under section 307(a) of the CWA. There are over 400 substances designated as either hazardous or toxic under the CWA.
• Any hazardous waste having the characteristics identified or listed under section 3001 of the Resource Conservation and Recovery Act.
• Any hazardous air pollutant listed under section 112 of the Clean Air Act, as amended. There are over 200 substances listed as hazardous air pollutants under the Clean Air Act (CAA).
• Any imminently hazardous chemical substance or mixture which the EPA Administrator has "taken action under" section 7 of the Toxic Substances Control Act.

Hazardous waste is defined under the Resource Conservation and Recovery Act (RCRA) as a solid waste (or combination of solid wastes) which, because of its quantity, concentration, or physical, chemical, or infectious characteristics, may: (1) cause or contribute to an increase in mortality or an increase in serious irreversible, or incapacitating illness; or (2) pose a substantial present or potential hazard to human health or the environment when improperly treated, stored, transported, disposed of, or otherwise managed. In addition, under RCRA, EPA establishes four characteristics that will determine whether a substance is considered hazardous, including ignitability, corrosiveness, reactivity, and toxicity. Any solid waste that exhibits one or more of these characteristics is classified as a hazardous waste under RCRA and, in turn, as a hazardous substance under Superfund.

The terms "hazardous substance" and "pollutant or contaminant" do not include petroleum or natural gas. EPA conducts emergency responses to incidents involving petroleum and non-petroleum oils separately from its responses to hazardous substance incidents. Throughout the Emergency Response Program, the term "hazardous substance" includes pollutants and contaminants.

In addition to the hazardous substances identified under the Superfund law, the Title III amendments to Superfund, also known as the Emergency Planning and Community Right-to-Know Act (EPCRA), identify several hundred hazardous substances for their extremely toxic properties. EPA designated them as "extremely hazardous substances" to help focus initial chemical emergency response planning efforts.