The document outlines the principles of environmental impact assessment (EIA) best practices according to the International Association for Impact Assessment (IAIA). It is divided into two parts. Part 1 describes the purpose and aims of developing EIA principles, which is to provide guidance to IAIA members and others applying EIA processes. It utilized results from previous EIA studies and expertise from IAIA members. Part 2 presents the definition of EIA, its objectives, and basic and operating principles for EIA. The basic principles include transparency, certainty, participation, practicality, flexibility, cost-effectiveness, credibility and accountability. The operating principles provide guidance on applying EIA at various stages such as screening, scoping and reporting.