Crop plants frequently face severe losses due to environmental stresses inflicted by various biotic agents, including fungi, bacteria, pests, nematodes, viruses, and other non-pest factors. In response to these challenges, plants activate their innate immunity by producing a repertoire of chemical compounds, such as jasmonates, salicylates, oxidative components, nitric oxide, and reactive oxygen species (ROS). Among these, ROS stand out as crucial functional signaling molecules that play multifaceted roles, encompassing not only the initiation of irreversible DNA damage and cell death but also the regulation of normal plant growth and stress responses. Under adverse conditions, plants generate a surplus of ROS species, orchestrating essential activities like programmed cell death, pathogen defense, and stomatal regulation. However, excessive ROS production under stress conditions can lead to abnormal cell death, disrupting regular metabolic processes. Consequently, plants have evolved specific chemical components and metabolic pathways aimed at curtailing ROS overproduction and safeguarding neighbouring cells from untimely demise. This book chapter underscores the significance of ROS in the context of plant biotic stress and delves into the intricate functioning of plants' antioxidant defense mechanisms in mitigating the detrimental impacts of oxidative stress, particularly under conditions of heightened stress. Furthermore, we propose that recent advancements in the field of epigenetics offer a promising avenue for further exploration, shedding light on the intricate roles played by ROS in bolstering biotic stress resistance in plants.