Many insect herbivores sequester (accumulate) plant defense metabolites in their bodies to utilize them for their own defense against natural enemies. In order to benefit from sequestered plant metabolites, an insect must take them up from the plant diet, avoid the metabolic breakdown to render them intact, avoid being auto-intoxicated during storage, and utilize them effectively for defense against opponents, e.g. by producing activating enzymes. Brassicales plants are equipped with a two-component chemical defense consisting of plant glucosides (glucosinolates) and separately stored activating enzymes (myrosinases). Upon tissue disruption, both components are mixed leading to the formation of toxic breakdown products (isothiocyanates). This thesis examines the physiological and biochemical mechanisms that allow the horseradish flea beetle, Phyllotreta armoraciae, to take up and store intact glucosinolates from its Brassicaceae host plants, and the effectiveness of glucosinolates for defense against predation. Investigations on the glucosinolate uptake from the plant diet showed that P. armoraciae prevents glucosinolate hydrolysis by plant myrosinases to a large extent. The rapid absorption of ingested glucosinolates separating the substrate from co-ingested enzymes and the inhibition of plant myrosinases in the gut are proposed to contribute to the sequestration process. Studies on the effectiveness of sequestered glucosinolates for defense revealed that the beetle relies on its host plant to obtain the glucosinolate substrates and on the simultaneous production of an own beetle myrosinase. Predation assays showed that this two-component chemical defense is life stage-specific since only selected contain both components to form toxic isothiocyanates for a successful defense and survival against a generalist predator. In summary, this thesis provides insights into the tritrophic interactions of P. armoraciae with its brassicaceous host plants and a predator.