Allelopathy is the study of biochemically-driven organismic interactions among primary producers. One organism affects others by the release of allelochemicals that are transported to the target cells, and cause a negative (or positive) response. Most aquatic allelochemicals are amphiphilic, thus have a sufficient solubility in the water, and at the same time can bind to and penetrate lipophilic cell membranes. Allelopathic interactions are not static but are influenced by variable environmental stressors. Resource availability can both affect the production and release of allelochemicals by the producing organism, but also influence the susceptibility of the target cells. The biosynthesis and excretion of allelochemicals might involve costs for the producing organism, and these costs will only be balanced if a net gain, i.e. better resource availability such as space or nutrients or secondary benefits, e.g. predator deterrence, are achieved. Allelopathic effects against cooccurring organisms might lead to coevolutionary responses, i.e. a lower susceptibility of target cells or to more advanced allelochemicals. Target organisms from different habitats might be more susceptible, especially if they are not acquainted with the allelochemicals. The transfer of laboratory results on allelopathy to realistic field conditions is complex, and might in the long run benefit from advanced analytical and molecular methods identifying specific target cell responses in situ.