This chapter explains that allelopathy is a complex process involved in the interaction of primary producers with other photosynthetic target organisms. It examines laboratory studies that allow the bioassay-directed fractionation of active compounds. Some of these studies say that environmental factors and the community composition can strongly affect the outcome of allelopathic interactions. It assesses whether the producer and target organisms have coevolved, and if allelopathy is more effective against species from the other habitats. It also states that a comparison of allelopathic interactions in different biomes shows some similarities in the chemical structure of released compounds and indicates common research themes, such as the efficiency by which allelochemicals affect neighbouring primary producers.