Plants have developed a wide variety of defense mechanisms to protect themselves against the attack of herbivorous insects. These mechanisms can be constitutive (always present) or inducible (activated only upon attack). Beside some general aspects of plant-herbivore interactions, the perception of herbivores by the infested host plants, the subsequent signal transduction processes as well as the initiation of selected defensive reactions will be presented. In particular, the contribution, the analysis, and the identity of mechanical as well as of chemical cues will be presented. Moreover, I will focus on the role of intracellular calcium changes as a signal at a very central position in the regulation of plant defense reactions. The Ca2+ signature of a given stimulus encodes a message that, after decoding by downstream Ca2+ sensors, contributes to a specific cellular response. We identified Calmodulin-like proteins that are involved in the regulation of insect-induced defense reactions. Their role and connection to the downstream jasmonate-depending pathways will be presented and discussed.