There has been rapid progress in detecting the genetic or allocation costs of induced resistance. In addition to these 'internal' costs, ecological costs may result from external mechanisms, that is, from the detrimental effects of resistance on the plant's interactions with its environment. All evolutionarily relevant costs affect a plant's ability to perform under natural conditions. The conceptual separation of different forms of resistance costs simplifies the study of mechanisms by which these costs arise. Yet, integrative measures of fitness must be applied under natural conditions so that researchers can fully understand the costs and benefits of induced resistance.