Brownfield sites are abandoned or underused industrial and commercial facilities available for re-use. Expansion or redevelopment of such a facility may be complicated by real or perceived environmental contaminations. Cf. Waste (law).

In the United States urban planning jargon, a brownfield site (or simply a brownfield) is land previously used for industrial purposes or certain commercial uses. The land may be contaminated by low concentrations of hazardous waste or pollution, and has the potential to be reused once it is cleaned up. Land that is more severely contaminated and has high concentrations of hazardous waste or pollution, such as a Superfund site, does not fall under the brownfield classification. Mothballed brownfields are properties that the owners are not willing to transfer or put to productive reuse.

In the United Kingdom and Australia, the term applies more generally to previously used land or to sections of industrial or commercial facilities that are to be upgraded.

The term brownfields first came into use on June 28, 1992, at a U.S. congressional field hearing hosted by the Northeast Midwest Congressional Coalition. Also in 1992, the first detailed policy analysis of the issue was convened by the Cuyahoga County Planning Commission. The United States Environmental Protection Agency selected Cuyahoga County as its first brownfield pilot project in September 1993.