The ability of plants to withstand herbivores relies on direct and indirect chemical defense. By using toxic phytochemicals, plants can deter and/or poison herbivores, while by releasing volatile organic compounds (VOCs) into the atmosphere plants can attract predators of the herbivores. The activation of specific responses requires recognition and appropriate response towards the attacking enemy and most of the events which finally lead to gene activation (the signaling pathway) occur within a few minutes. Among the several signaling molecules involved, reactive oxygen species (ROS) and intracellular calcium signatures belong to early events, which are responsible for most of the ensuing cascades of chemical and molecular reactions. In this review, we will focus on rapid early events following insect feeding on plants that eventually lead to the production and release of phytochemicals. Furthermore, we compare and discuss the impact of mechanical and biotroph wounding.