Sustainability Impact Assessment (SIA) is an increasingly accepted form of strategic impact assessment, and has recently been adopted, for example, as the preferred method of ex ante policy assessment in the European Community. A methodology for SIA for application to international trade policy measures was developed by IDPM in 1999, and since then has been used in conducting an ongoing SIA study of the WTO trade negotiations agenda. This paper will review the past four years' experience in carrying out SIA of trade policy, and will identify the main difficulties and challenges that have arisen in its application. The main lessons for the further development of the methodology will then be discussed.