The attempt to rationalize the connections between the molecular structures of organic compounds and their biological activities comprises the field of structure-activity relations (SAR) studies. Correlations between structure and activity are important for the understanding and the development of pharmaceutical agents, agricultural agents, and chemical communicants (olfactory and gustatory stimulants) and for the investigation of chemical and genetic toxicity (mutagenic and carcinogenic potential). Practical importance attaches to this type of study because of potential exists for prediction of the biological activity of untested or even hypothetical compounds. The techniques of SAR have been applied in the pharmaceutical field (drug design) and, to some extent, in agricultural chemicals. The methods are beginning to be applied to problems in the areas of chemical and genetic toxicity.