In the United States, an environmental site assessment is a report prepared for a real estate holding that identifies potential or existing environmental contamination liabilities. The analysis, often called an ESA, typically addresses both the underlying land as well as physical improvements to the property. A proportion of contaminated sites are "brownfield sites." In severe cases, brownfield sites may be added to the National Priorities List where they will be subject to the U.S. Environmental Protection Agency's Superfund program.
The actual sampling of soil, air, groundwater and/or building materials is typically not conducted during a Phase I ESA. The Phase I ESA is generally considered the first step in the process of environmental due diligence. Standards for performing a Phase I site assessment have been promulgated by the US EPA  and are based in part on ASTM in Standard E1527-13.
Phase II Environmental Site Assessment is an "intrusive" investigation which collects original samples of soil, groundwater or building materials to analyze for quantitative values of various contaminants. This investigation is normally undertaken when a Phase I ESA determines a likelihood of site contamination. The most frequent substances tested are petroleum hydrocarbons, heavy metals, pesticides, solvents, asbestos and mold.