Plants communicate and influence the growth of other plants (or even microorganisms) through excretion of certain chemical compounds (allelochemicals). The process is called allelopathy. A number of allelochemicals have been reported from different plant species. Most important allelochemicals/allelochemical groups in major field crops (those focused in this book) may include phenolic compounds, benzoxazinoids, sorgoleone, glucosinolates, terpenes, alkaloids, and momilactones. The allelopathic potential of field crops may be utilized for controlling weeds without importing weed control agent into the field. This is possible through channelizing the allelopathic activity of field crops for controlling weeds in form of several techniques. Most importantly, these techniques may include growing the crop cultivars that possess an allelopathic potential. The other important ways may include intercropping of a crop possessing an allelopathic potential with a crop without allelopathic activity [e.g., intercropping of sorghum (Sorghum bicolor (L.) Moench) in cotton (Gossypium hirsutum L.)], including a crop possessing an allelopathic activity in a crop rotation, use of residues from an allelopathic crop as mulch, and use of an allelopathic crop as cover crop for controlling weeds. Most important in future research include determining the mode of action of allelochemicals and their formulation into a commercial weed control product.