The present book is an attempt to study the most exciting phenomenon of plant-plant interaction, known as Allelopathy. This book enables its readers to get acquainted with the details of allelopathic effect of Tridex procumbens, a common weed on the growth performance of four important crop plants (Vigna aconitifolia, Lepidium sativum, Trigonell foenumgraecum and Cicer arietinum). The effect of T. procumbens plant extract was studies at both in vitro and in vivo level. Three different concentrations of the plant extraxt (1%, 50% and 100%) was used at both levels in triplicate along with the control. After 7 days of incubation at room temperature maximum reduction in percent seed germination (30.00±10.00) was reported in with Vigna aconitifolia at 100% concentration at in vitro level. Maximum reduction in germination index (21.86± 2.54) in the same plant with same concentration. Similar trend (reduction in %G=40.00± 20.00 and GI=17.3± 3.63) was reported at in vivo level.