The current online help is aimed at providing essential information on using the OECD QSAR Toolbox. The QSAR Toolbox is a project of the Organisation for Economic Co-Operation and Development in collaboration with the European Chemical Agency. It has been developed by the Laboratory of Mathematical Chemistry. The main objective of the Toolbox is to allow the user to use (Q)SAR methodologies to group chemicals into categories and to fill data gaps by read-across, trend analysis and (Q)SARs. For in-depth background information on the concept of chemical categories, the user is invited to consult the guidance document for grouping of chemicals published in the Series on Testing and Assessment of the OECD Environment, Health and Safety Publications [OECD (2007); ENV/JM/MONO(2007)28: http://www.oecd.org/officialdocuments/displaydocument/?doclanguage=en&cote=env/jm/mono(2007)28]. Additional guidance and training material are available on the dedicated internet site for the QSAR Toolbox [http://www.qsartoolbox.org], the internet site for the OECD (Q)SAR Project [http://www.oecd.org/env/existingchemicals/qsar] as well as the internet site of the developer of the QSAR Toolbox [http://toolbox.oasis-lmc.org/]. The user is invited to regularly consult and these internet sites.