Allelopathy, initially defined as the harmful or stimulatory biochemical interaction of one plant on another through the release of secondary compounds (natural products, which do not function directly in the primary biochemical activities that support the growth, development, and reproduction of the plant in which they occur), 1 is now generally accepted to include plant/insect and plant/pathogen interactions where secondary plant-defense compounds are involved. The potential role of allelopathy in agriculture has been extensively reviewed. 2-4 While many secondary plant compounds have been described to be involved in allelopathy in nature, many more have been demonstrated to have pesticidal activity. 5-9 In evaluating the effects of agricultural chemicals on allelopathy in crop systems, it is useful to consider the effects on secondary plant metabolism in general. Therefore, although this review emphasizes the effects of agricultural chemicals on secondary products in crop systems, examples of agricultural chemical effects on secondary metabolism in noncrop systems are also included. In addressing the pesticide effects on allelopathy of plants, this review will adopt the broader definition of allelopathy and will consider each agricultural chemical group separately.