The project “Application of the Allelopathic Effect in Plant Cultivation Technologies” was designed to investigate the impact of allelochemical compounds secreted by plants on agricultural crops and their potential in creating more sustainable agrosystems. The research focused on analyzing the allelopathic properties of three major plant families—legumes (Fabaceae), grasses (Poaceae), and crucifers (Brassicaceae)—and their practical application in multi-species crops, cover crops, and mulching technologies. The project’s goal was to contribute to environmental protection and climate change mitigation by reducing the need for chemical pesticides. Allelopathy is defined as a biochemical interaction between plants and other organisms in which substances released can inhibit or promote the germination, growth, and development of other plants. Studies have shown that allelochemical compounds—particularly phenolic compounds and glucosinolates—have a significant impact on physiological processes such as photosynthesis, respiration, and nutrient uptake. These properties allow plants to be used as a natural means of controlling weeds, diseases, and pests, thereby reducing the use of synthetic pesticides. Research has shown that the allelopathic effects of different plant species vary greatly and depend on the plant species, growing conditions, and the concentration of allelochemicals. In Lithuania, certain weeds, such as field bindweed and common ragweed, exhibit strong allelopathic effects. Buckwheat, rapeseed, and other bast fiber plants also have a significant impact, as the compounds present in their residues can inhibit weed growth. Particularly important are rapeseed glucosinolates and their hydrolysis products (isothiocyanates), which possess herbicidal, fungicidal, and bactericidal properties and can be used in biofumigation. Laboratory and laboratory-field experiments have shown that extracts of organic mulches have varying effects on plants. For example, radish germination was most often promoted, while that of carrots and lettuce was more frequently inhibited. Straw and sawdust mulches promoted root and seedling growth, but the effect depended on the concentration of the extracts. This confirms that the effect of allelopathic compounds is complex and can be both positive and negative.