As a general approach to examining relationships between structure and biological activity, the classical QSAR (quantitative structure-activity relationship) procedure was published some 55 years ago by Corwin Hansch and his coworkers including the present author (Hansch et al., 1963; Hansch & Fujita, 1964). The original project at that time was to get an insight into (sub)molecular mechanism of action of the plant growth regulators/herbicides of substituted phenoxyacetic acid series through analyzing substituent effects on their structure-activity relationship (Hansch et al., 1963). Thus, the classical QSAR of the Hansch-type stems from agrochemistry. Since then, a number of the classical QSAR studies have been published not only in agrochemical (Draber & Fujita, 1992; Hansch & Fujita, 1995) but also more in medicinal/pharmaceutical chemistry area (Fujita, 1990; Hansch & Leo, 1995). We could expect two categories of consequences from analyses of the structure-activity relationships for series of bioactive compounds. One is to extract guiding principles and construct models to design and synthesize new compounds the activity profiles of which are improved over those of existing series members. The second is to obtain an insight into the (sub)molecular mechanism of the biological action. In this article, examples of agrochemical QSAR studies from two categories carried out in some Japanese institutions are reviewed.