Isoprene and other terpenoids are important biogenic volatile organic compounds in terms of atmospheric chemistry. Isoprene can aid plant performance under abiotic stresses, but the fundamental biological reasons for the high emissions are not completely understood. Here, we provide evidence of a previously unrecognized ecological function for isoprene and for the sesquiterpene, ß-caryophyllene. We show that isoprene and ß-caryophyllene act as core components of plant signaling networks, inducing resistance against microbial pathogens in neighboring plants. We challenged Arabidopsis thaliana with Pseudomonas syringae, after exposure to pure volatile terpenoids or to volatile emissions of transformed poplar or Arabidopsis plants. The data suggests that isoprene induces a defense response in receiver plants that is similar to that elicited by monoterpenes and depended on salicylic acid (SA) signaling. In contrast, the sesquiterpene, ß-caryophyllene, induced resistance via jasmonic acid (JA)-signaling. The experiments in an open environment show that natural biological emissions are enough to induce resistance in neighboring Arabidopsis. Our results show that both isoprene and ß-caryophyllene function as allelochemical components in complex plant signaling networks. Knowledge of this system may be used to boost plant immunity against microbial pathogens in various crop management schemes.