The objective of this document is to provide guidance to risk assessors on the use of quantitative toxicokinetic and toxicodynamic data to address interspecies and interindividual differences in dose/concentration-response assessment. Section 1 focuses on the relevance of this guidance in the context of the broader risk assessment paradigm and other initiatives of the International Program on Chemical Safety (IPCS) project on the Harmonization of Approaches to the Assessment of Risk from Exposure to Chemicals. Technical background material is presented in section 2, followed by generic guidance for the development of chemical-specific adjustment factors in section 3 and accompanying summary figures. Illustrative case-studies are included in an Appendix and a glossary of terms is also provided.