Research on the physiology of crops under stress is of relatively recent origin and still in the stage of information building; practical use of the information gathered is yet to be made. Research on allelopathy and use of allelochemicals is one such example. The majority of work done focuses on the detrimental effects of living plants and their residues on crop growth, yield and quality, and little attention has been paid, that too only in the last 15-20 years, on making gainful use of this nature's self defence mechanism, both by crops for weeds and weeds for crops, and so on. In this volume, unlike other books and symposium-volumes, efforts have been made to understand the problem and concentrate on aspects such as autotoxicity, crop-crop interference, crop-weed interference, weed-crop interference etc. for better understanding of the problems of monoculture, ratooning, crop rotation, mixed or strip cropping, mulching and weed smothering, etc. Further, the possibility of developing environment-friendly bio- or synthetic herbicides, and developing more aggressive crop varieties that can smother, kill or tolerate noxious weeds through conventional breeding or by using modern genetic engineering techniques to transfer desirable genes into commercial crop varieties, has been considered. Efforts made for identifying gene markers and modeling have also been discussed.