Plants defend themselves against attack by herbivores with a variety of physical and chemical defences, some of which also work by recruiting partners from the third trophic level. Despite the known or potential benefits of possessing defence traits, the expression of defence traits varies among tissues within individual plants, within and among plant populations, and across biotic and abiotic environments. The central explanation for such variation is that the expression of defence traits is costly to fitness in the absence of benefits. Here, we review how cost-benefit trade-offs have been incorporated in several hypotheses about the ecology and evolution of plant defences. We then describe several approaches that have been used to examine costs and the empirical evidence that has been attained through their use. After 30 years of study, a consensus has emerged through a variety of approaches that expression of defence traits is indeed costly but that the magnitude and importance of costs are context-dependent.