In her paper Environmental Impact Assessment (EIA) Claire Pettit of the Institute of Environmental Management and Assessment provides the following introduction to the Environmental Impact Assessment process:
"EIA was introduced to address concerns regarding the effects that major development projects were having on the environment. Prior to the introduction of EIA although major development proposals underwent an appraisal process, the appraisal did not consider environmental factors and therefore one of the initial aims of EIA was to redress this balance. EIA was first introduced in the USA in 1969. The benefits associated with the process were quickly recognised and within 20 years EIA had become established world wide as an important environmental decision making tool. EIA was first introduced into the European Union as a directive in 1985."

The Environmental Impact Assessment process was introduced into the European Union in 1985 as Council Directive 85/337/EEC ('the EIA Directive') to ensure that environmental concerns are assessed and considered as part of the development consent process.

Article 2(1) of Directive 85/337/EEC states:
"Member States shall adopt all measures necessary to ensure that, before consent is given, projects likely to have significant effects on the environment by virtue, inter alia, of their nature, size or location are made subject to an assessment with regard to their effects."

The EIA Directive was subsequently amended in 1997 (97/11/EC), 2003 (2003/35/EC) and 2009 (2009/31/EC). Generally, the UK has been slow to embrace the EIA Directives, and has often failed to implement the necessary domestic legislation in a timely manner. On occasion, the UK has also failed to interpret the EIA Directives correctly, resulting in domestic legislation that, from time to time, did not comply with the EIA Directives. As of September 2011, the Welsh Government has failed to introduce implementing Regulations in respect of Directive 2009/31/EC by the applicable deadline (25 June 2011).

In England and Wales, the EIA Directives are usually applied through the Town and Country Planning (Environmental Impact Assessment) (England and Wales) Regulations 1999 - SI 1999 No 293 ('the EIA Regulations'). An update to these EIA Regulations is expected soon, in order to update UK law in line with the EIA Directives and EIA case law.

The EIA Regulations apply to the assessment of the effects of certain public and private projects on the environment, and guidance for local authorities in Wales was published in 1999 through Welsh Office Circular 11/99.

New developments as well as changes to existing developments likely to have significant effects on the environment can be subject to the EIA process. The EIA process ensures that the potential effects on the environment are considered, including impacts on air, land, and water; protection of habitats; and impacts upon the local community. EIA provides a mechanism for predicting the interaction of environmental effects arising from the development, allowing them to be avoided or reduced through the development of mitigation measures. By this means, EIA contributes to decision-making at several stages in the development process.

EIA steers the whole life cycle of a development, including site investigations; risk assessments; project design; mitigation measures; development plans; and site monitoring. EIA is much more than remediation, and inspection of the result. And it applies to the entire development and, potentially, other developments nearby so that, for example, the combined detrimental effects of several projects in a given locality do not, in combination, result in significant adverse effects upon the environment.

For more general information about EIA, see the Wikipedia entry for 'Environmental impact assessment'.