This review discusses recent progress in our understanding of signaling in induced plant resistance and susceptibility to pathogens and insect herbivores, with a focus on the connections and crosstalk among phytohormone signaling networks that regulate responses to these and other stresses. Multiple stresses, often simultaneous, reduce growth and yield in plants. However, prior challenge by a pathogen or insect herbivore also can induce resistance to subsequent challenge. This resistance, or failure of susceptibility, must be orchestrated within a larger physiological context that is strongly influenced by other biotic agents and by abiotic stresses such as inadequate light, temperature extremes, drought, nutrient limitation, and soil salinity. Continued research in this area is predicated on the notion that effective utilization of induced resistance in crop protection will require a functional understanding of the physiological consequences of the "induced" state of the plant, coupled with the knowledge of the specificity and compatibility of the signaling systems leading to this state. This information may guide related strategies to improve crop performance in suboptimal environments, and define the limits of induced resistance in certain agricultural contexts.