The idea that the concentration of secondary metabolites in plant tissues is controlled by the availability of carbon and nitrogen in the environment has been termed the carbon–nutrient balance hypothesis (CNB). This hypothesis has been invoked both for prediction and for post hoc explanation of the results of hundreds of studies. Although it successfully predicts outcomes in some cases, it fails to such an extent that it cannot any longer be considered useful as a predictive tool. As information from studies has accumulated, many attempts have been made to save CNB, but these have been largely unsuccessful and have managed only to limit its utility. The failure of CNB is rooted in assumptions that are now known to be incorrect and it is time to abandon CNB because continued use of the hypothesis is now hindering understanding of plant–consumer interactions. In its place we propose development of theory with a firm evolutionary basis that is mechanistically sophisticated in terms of plant and herbivore physiology and genetics.