The present publication is an updated version of the IAEA Assessment of Safety Significant Events Team (ASSET) Guidelines, IAEA-TECDOC-573, published in 1990. Sections 5 and 6 include revised definitions and investigation guidelines for identification of both direct and root causes. These revisions were recommended by a Consultants Meeting held in Vienna on 3-7 December 1990. This guidance is not intended to infringe an expert`s prerogative to investigate additional items. Its main purpose is to provide a basic structure and ensure consistency in the assessments. Use of the ASSET guidelines should also facilitate comparison between the observations made in different nuclear power plants and harmonize the reporting of generic ASSET results. The guidelines should always be used with a critical attitude and a view to possible improvements.