The aims and objectives of the research project were to: identify the characteristics and nature of the way in which 'significant environmental effects' are currently determined and articulated under the Town and Country Planning (Environmental Impact Assessment) Regulations, 1999 (EIA); explore the use of 'fuzzy set' theory to understand the way in which significance is evaluated by a range of stakeholders at different EIA decision stages leading up to the submission of an Environmental Impact Statement (EIS) for final project authorisation decision-making; theorise the evaluation of the significance of environmental effects and develop heuristic models of EIA information appraisal and decision-making in the stages leading up to the submission of EISs; and evaluate the extent to which fuzzy sets may be employed to develop improvements in procedures used to evaluate significant environmental effects in screening, scoping and impact prediction in EIA. The data relate to a wide range of screening and scoping practice in EIA in England and Wales. Responses are provided separately for three groups; local planning authorities, environmental consultants and statutory consultees involved in the EIA process. Data relate to practitioner experience of screening and scoping, approaches used to determine impact significance and perceived constraints on practice.