This paper selectively reviews developments in the study of allelopathy, principally from a chemical standpoint. Particular attention is given to recently discovered aspects of signalling within the rhizosphere rather than to direct physiological effects caused by broadly active allelopathic agents with only a brief review of the role of bulk generation of weakly biocidal material. Many of the chemicals responsible for allelopathic effects have been identified and the biosynthesis and modes of action of a number are included. The paper also highlights instances where signalling aspects of allelopathy may be exploited for practical weed control purposes, and how the study of allelopathic components of the rhizosphere metabolome might be studied more directly by means of plant genomics.