Cytokinins (CK), a group of plant hormones, are well known for their functions in growth and development as well as in environmental interactions. Except for some prominent examples like green islands and leafgalls caused by endophytic insects via CK, only little is known about the function of CK in plantinsect interaction. In my thesis I have studied the role of CK in the ecological model plant Nicotiana attenuata and two of its most abundant specialized free living herbivores, the larvae of the tobacco hawkmoth Manduca sexta as well as the mirid bug Tupiocoris notatus. I studied the effect of herbivory on CK levels and CK pathway and used transgenic plants with altered CK biosynthesis or perception to demonstrate the effect of CK on expression of herbivore induced defenses (HID). HID follow a developmentally dependent concentrationgradient predicted by the Optimal Defense Theory: High concentrations of HID in young leaves with a higher value for the plants reproductive success, and low levels in old leaves. We observed that CK follow the same gradient and that increasing CK levels in old leaves was sufficient to increase HID levels in old leaves to levels comparable to young leaves. However, this increase in HID did not improve the plants resistance against herbivory. This is possibly due to CK simultaneously inhibiting senesce processes which might be an important part of a plants defense against specialized herbivores. T. notatus seems capable to actively manipulate these senescence processes via CK and stabilize nutrient levels in attacked leaves. With 15N labeled plants we could prove that T. notatus transfers the CK N6isopentenyladenine to its hostplant via its oral secretions. A phenomenon so far not described for free living insects. Based on my results I suggest that the role of CK goes far beyond the known cases of endophytes and that CK have rather a key role in the interaction of plants and insects.