Induced responses to herbivores are common and well documented in plants. It has been hypothesized that the evolutionary ecology of induced responses can be understood by studying benefits of induction in the presence of herbivores and costs of induction in the absence of herbivores. Phenotypic benefits and costs of induction would indicate that such plasticity in defense could be adaptive (i.e., that phenotypes matched to their environmental conditions have higher relative fitness than unmatched phenotypes). However, few studies to date have investigated the benefits and costs of induction in the same system. In this study, induced responses of Lepidium virginicum to herbivory reduced feeding by generalist noctuid caterpillars in choice and no?choice experiments. Induced plant responses to herbivory were correlated with an increase in the number of trichomes per leaf and an increase in the diversity of the putatively defensive chemical compounds, glucosinolates, present in the foliage of damaged plants compared to undamaged controls. Induction did not affect the feeding behavior of the larvae of the specialist butterfly, Pieris rapae. In field experiments, induction reduced natural colonization of plants by aphids compared to both unmanipulated controls and controls that were damaged (but not induced) by clipping a leaf from the plant using a pair of scissors. Induced plants were more likely to survive in the field than clipped plants, a result that suggests a net fitness benefit of induction when leaf tissue removal was controlled. In experiments conducted in the absence of herbivores, damage induced responses did not reduce the root or shoot biomass of plants grown at low density. At high plant density, induction was associated with both reduced root biomass and increased aboveground growth, suggesting that induction may cause an allocation shift, rather than a loss of total biomass. Induced responses of plants satisfy a necessary component of adaptive plasticity because plants in variable herbivore environments maximize relative fitness by adjusting their defensive phenotype.