Fitness costs can explain the evolution and maintenance of induced resistance in plants. However, the methods currently used to gather evidence for such costs do not allow decreases in fitness to be cleanly attributed to individual traits responsible for induced resistance, because the studies are plagued by multiple confounding responses. Reproductive performance provides an overview of physiological performance and is thus a useful currency to understand the function of induced responses. Integrated molecular, physiological and ecological studies are needed to identify the mechanisms responsible for the decrease in fitness and to evaluate fully the usefulness of the cost paradigm.