This guidance document is part of the OECD effort to provide guidance for assessing the hazards of chemical substances while gaining efficiencies and improving animal welfare. The approach described in this guidance document is to consider closely related chemicals as a group, or category, rather than as individual chemicals. While the first edition was published in 2007, This edition has been augmented with experience and examples encountered in the OECD Cooperative Chemicals Assessment Programme, formerly the HPV Chemicals Programme since 2007, the second edition also intends to introduce new or revised guidance on: elaborating the analogue and category approach, quantitative and qualitative read-across, justifying read-across, using bioprofiling results for grouping chemicals, and specific types of category approaches (e.g. chemicals of variable composition, and metals).