The number of papers published on allelopathy was low until the 1970s but has accelerated rapidly in recent years. Alleged allelopathic activity has been suggested for numerous weed and crop species; however, many studies employed preliminary bioassays (leachates or extracts in the absence of soil). The importance of substratum and of abiotic and biotic stresses (nutrients, shade, herbides, plant diseases, herbivores), plant density, habitat, and climate has been realized in recent years. Laboratory bioassay, the first step to investigate probable involvement of allelopathy, should be designed to correspond to field conditions. Studies need to focus on the effects of (1) climatic, environmental, and habitat factors on allelopathic potential; (2) abiotic and biotic soil components on fate and biological activity of allelochemicals; and (3) allelochemicals on soil nutrient dynamics, microbial ecology, and other abiotic and biotic components. Recently, attempts to find crop cultivars with a competitive allelopathic edge have been made. Interaction of professionals from different disciplines is needed to understand the complexity of the ecosystem. The following are important topics for discussion: allelopathy and the transition to sustainable agriculture; physiological and biochemical studies; using transgenes to produce allelopathic crops; microbial allelochemicals and pathogens as weed biological agents; bioassays for allelopathy (problems and solutions); pollen allelopathy; and an ecological perspective of allelopathy.