Allelopathy interactions are based primarily on the production of secondary chemicals by higher plants that produce an array of biochemical compounds that create biological changes. This international collection of 16 papers shares current thought about the chemistry and mode of action of these compounds and how they affect ecology, agriculture, and forests. Topics include the role of lichen secondary metabolites, analytical techniques for investigating allelopathic agents in wheat root exudates, the fate of phenolic allelochemicals in soils, allelochemical properties of quinolizidine alkaloids, and the mode of action of phytotoxic terpenoids.