The defensive roles of reactive oxygen species (ROS) especially under biotic stress by a fungal infection are discussed in this book. The latest researches and reviews on the essential role played by ROS accumulation in the cellular environment are amalgamated to get the latest vision into the oxidative burst. It has been further extended to the case study on Verticillium dahliae on many commercially significant cultivars and ROS mediated defense mechanisms employed by the different plants against this fungus. It is focused on the latest findings of ROS being a defense agent for plants. These understandings will enable the application of latest biological techniques in extrapolating the plant genomes for better protection based on enhancing natural defensive actions such as ROS mediated mechanisms that may prove to be a savior for huge troll of crop yield losses globally. This entire piece of work will be broad of interest to the people from the arrears of horticulture, botany, biochemistry, biotechnology, and plant sciences.