Plant-herbivore interactions are of immense ecological and evolutionary significance in natural and managed ecosystems. Herbivores consume over 15 percent of the plant biomass annually produced in temperate and tropical ecosystems, and herbivory costs world economies billions of dollars in lost agricultural and silvicultural revenue. Thus, a basic understanding of the evolutionary ecology of plant-insect interactions is not only required to understand how ecosystems function, but also how humans can live sustainably in their environment. This symposium seeks to answer a long-standing question in the study of plant-insect interactions: why do plant species vary in their levels and effectiveness of defenses against insect herbivores? Despite decades of theory and experiments, the answer to this problem remains elusive. This symposium highlights the research of seven leading groups seeking to understand the evolutionary ecology of plant defenses against herbivores. Their research integrates the most recent theoretical, analytical, and technological advances to understand the ecology and evolution of plant-insect interactions over micro- and macroevolutionary timescales. The symposium starts with a review of classic theories and assumptions of the evolutionary ecology of plant defense (Agrawal). This is followed by talk that employ comparative phylogenetic approaches and experiments to try understand how the evolution of plant defenses over macroevolutionary timescales in tropical (Coley and Kursar) and temperate (Johnson et al.) ecosystems leads to variation in defense among plant species. The first half is concluded with an examination of latitudinal gradients in herbivory and resistance around the world (Moles). The second half of the symposium focuses on microevolutionary approaches to the study of plant defense evolution and ecology. We will first learn how plants evolve resistance to herbivores in the ecologically simplified context of islands (Stenberg) and whether incorporating variation at the third trophic level helps to explain variation in the strength of selection on plant defense (Haloin and Strauss). The symposium will conclude with an examination of whether it is possible to detect selection on specific genes putatively involved in herbivore resistance in the field (Mitchell-Olds). Together these talks offer a modern synthesis of our understanding of the evolutionary ecology of plant defense and why variation in plant defense is expected to be the rule, not the exception.