Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial (positive allelopathy) or detrimental (negative allelopathy) effects on the target organisms and the community. Allelopathy is often used narrowly to describe chemically mediated competition between plants; however, it is sometimes defined more broadly as chemically mediated competition between any type of organisms. The original concept developed by Hans Molisch in 1937 seemed focused only on interactions between plants, between microorganisms and between microorganisms and plants. Allelochemicals are a subset of secondary metabolites, which are not directly required for metabolism (i.e. growth, development and reproduction) of the allelopathic organism.

Allelopathic interactions are an important factor in determining species distribution and abundance within plant communities, and are also thought to be important in the success of many invasive plants. For specific examples, see black walnut (Juglans nigra), tree of heaven (Ailanthus altissima), black crowberry (Empetrum nigrum), spotted knapweed (Centaurea stoebe), garlic mustard (Alliaria petiolata), Casuarina/Allocasuarina spp., and nut grass (Cyperus rotundus).

Allelopathy is classified as a biotic factor, as it involves chemical interactions between living organisms, most commonly among plants. In allelopathic interactions, certain species release chemical compounds into the environment that inhibit the germination, growth, or reproduction of neighboring organisms. This process provides a competitive advantage to the allelopathic species by directly interfering with the development of potential competitors.

Allelopathy is frequently mistaken for resource competition, another biotic factor in which organisms compete for limited abiotic resources such as sunlight, water, and soil nutrients. However, the two processes are functionally distinct. While allelopathy involves the introduction of inhibitory chemical agents into the environment, resource competition results from the depletion of essential environmental resources. In many ecological contexts, both forms of competition may operate concurrently, complicating efforts to isolate the specific contribution of allelopathy.

Further complexity arises from the fact that certain allelochemicals may indirectly limit resource availability, thereby mimicking the effects of resource competition. Additionally, the production and efficacy of allelochemicals are influenced by a range of environmental variables, including nutrient availability, temperature, and soil pH. Although the existence of allelopathy is widely accepted in ecological literature, individual cases often remain contentious. Moreover, the specific physiological and ecological mechanisms through which allelochemicals affect target species are still the subject of ongoing research.

The term allelopathy from the Greek-derived compounds allilon- (αλλήλων) and -pathy (πάθη) (meaning "mutual harm" or "suffering"), was first used in 1937 by the Austrian professor Hans Molisch in the book Der Einfluss einer Pflanze auf die andere - Allelopathie (The Effect of Plants on Each Other - Allelopathy) published in German. He used the term to describe biochemical interactions by means of which a plant inhibits the growth of neighbouring plants. In 1971, Whittaker and Feeny published a review in the journal Science, which proposed an expanded definition of allelochemical interactions that would incorporate all chemical interactions among organisms. In 1984, Elroy Leon Rice in his monograph on allelopathy enlarged the definition to include all direct positive or negative effects of a plant on another plant or on micro-organisms by the liberation of biochemicals into the natural environment. Over the next ten years, the term was used by other researchers to describe broader chemical interactions between organisms, and by 1996 the International Allelopathy Society (IAS) defined allelopathy as "Any process involving secondary metabolites produced by plants, algae, bacteria and fungi that influences the growth and development of agriculture and biological systems." In more recent times, plant researchers have begun to switch back to the original definition of substances that are produced by one plant that inhibit another plant. Confusing the issue more, zoologists have borrowed the term to describe chemical interactions between invertebrates like corals and sponges.

Long before the term allelopathy was used, people observed the negative effects that one plant could have on another. Theophrastus, who lived around 300 BCE noticed the inhibitory effects of pigweed on alfalfa. In China around the first century CE, the author of Shennong Ben Cao Jing, a book on agriculture and medicinal plants, described 267 plants that had pesticidal abilities, including those with allelopathic effects. In 1832, the Swiss botanist De Candolle suggested that crop plant exudates were responsible for an agriculture problem called soil sickness.

Allelopathy is not universally accepted among ecologists. Many have argued that its effects cannot be distinguished from the exploitation competition that occurs when two (or more) organisms attempt to use the same limited resource, to the detriment of one or both. In the 1970s, great effort went into distinguishing competitive and allelopathic effects by some researchers, while in the 1990s others argued that the effects were often interdependent and could not readily be distinguished. In 1994, D. L. Liu and J. V. Lowett at the Department of Agronomy and Soil Science, University of New England in Armidale, New South Wales, Australia, published two papers in the Journal of Chemical Ecology that developed methods to separate the allelochemical effects from other competitive effects, using barley plants and inventing a process to examine the allelochemicals directly. In 1994, M.C. Nilsson at the Swedish University of Agricultural Sciences in Umeå showed in a field study that allelopathy exerted by Empetrum hermaphroditum reduced growth of Scots pine seedlings by ~ 40%, and that below-ground resource competition by E. hermaphroditum accounted for the remaining growth reduction. For this work she inserted PVC-tubes into the ground to reduce below-ground competition or added charcoal to soil surface to reduce the impact of allelopathy, as well as a treatment combining the two methods. However, the use of activated carbon to make inferences about allelopathy has itself been criticized because of the potential for the charcoal to directly affect plant growth by altering nutrient availability.