During their lifetime, plants need to adapt to various stimuli originating from the abiotic and biotic environment, some of which represent stress factors. One major biotic stress factor is the attack of herbivorous insects feeding on the plant. But plants are not defenseless; they are equipped with an arsenal of different defense layers. Additionally to mechanical barriers, which are a first line of defense, the plant can produce a variety of chemicals like toxic secondary metabolites, anti-herbivory proteins or compounds involved in indirect defense. The production of many of these defensive compounds is triggered by HAMPs, elicitors in insects’ oral secretions (OS), which come in contact with the wounded plant tissues while insects are feeding. The early events upon perception of these stimuli are still poorly understood. Elevations in cytosolic calcium are one of these early events, which activate the downstream defense signaling network, including certain phytohormones. To reach this, a proper decoding of calcium signals by different calcium sensor proteins is important. In Arabidopsis thaliana, several members of the calmodulin-like proteins (CMLs), one group of calcium sensor proteins, is induced upon treatment with OS of the generalist herbivore Spodoptera littoralis. Some of these CMLs are involved in herbivore defense by modulating the jasmonate pathway.