Allelopathy is a biological phenomenon where one plant inhibits the growth and development of another plant through the release of chemicals in the environment. In essence, plant allelopathy is used as a means of survival in nature, reducing competition from plants nearby. Allelochemicals in fact are secondary metabolites, which are not required for metabolism of the allelopathic organism. Phenols are the most important category of the many secondary metabolites implicated in plant allelopathy. Phenolic compounds arise from the shikimic and malonat acid metabolic pathways in plants. There are many interesting examples of allelopathy: allelopathy as a mechanism for the invasion of Typha angustifolia, allelochemicals released from rice plants, allelochemicals released from black walnuts and others. From an agronomic point of view, allelopathy is important because some weed plants like rice are used as natural herbicids against crop production. Apart from target plants, nontarget plants are affected by allelopathy too. Other ecological effects are evident through inhibition of their microbial symbionts such as mycorrhiza and nitrogen fixing bacteria, changes in population and community structure, then changes in genotypic variation in plant populations, invasive plants etc.