Allelopathy phenomenon describes both positive and negative interactions between plants through chemicals released into environment. Inhibition of growth of various organs and delay or restriction of seed germination are most frequently observed effects of toxicity of allelochemicals, although some of them, when applied in minimal concentrations, lead to the effect of hormesis (stimulation of seed germination or seedling growth). Plants subjected to some allelopathic compounds respond by induction of oxidative stress, manifested as overproduction of reactive oxygen species (ROS), alterations in cellular antioxidant system (activity of catalase, superoxide dismutase, and enzymes of Halliwell–Asada–Foyer cycle, the level of ascorbate and glutathione). Due to differences in plant sensitivity to allelochemicals, and considering a wide spectrum of concentration of already tested compounds, it is hardly to believe that induction of oxidative stress is a global and uniform reaction to all allelopathins. There are several examples of allelopathic compounds that act as antioxidant rather then prooxidant agents. In the chapter we present the data describing induction of alterations in ROS production or/and metabolism in acceptor plants by various allelochemicals or extracts of other plants releasing allelochemicals into environment. Special attention is paid on oxidative deterioration induced by essential oils. Accumulation of ROS and modification of growth and/or development of organs of acceptor plants as induced by allelotoxins were correlated with “stress induced morphogenic responses” (SIMRs), modification of phytohormonal balance and factors crucial for cell wall formation. It was proposed a link between phytotoxicity of allelochemicals and activation of ROS-mediated transduction pathways, involving also calcium dependent intracellular signaling.