This monograph has been prepared on behalf of the Central Unit of the International Programme on Chemical Safety (IPCS) and its aim is to provide an update of the principles utilized by the WHO Expert Group on Pesticide Residues. It does not address the work of the FAO Panel.The more recent additions to the battery of toxicity tests avail-able for use in safety assessment are discussed in this monograph.Each chapter in this monograph provides a rational background to a specific area, describes the history of relevant changes in principles according to the development of scientific knowledge, and offers a short summary of the current position of JMPR. It also indicates the principles being followed at present by the WHO Expert Group in thefr evaluations of pesticide residues in food and recommendations on how the studies may be performed to provide meaningful results.