Why do leaves have such varied sizes and shapes? Part of the answer lies in physiological and biomechanical demands imposed by different habitats; selective forces that are now reasonably well understood. In contrast, the impact of herbivores on the evolution of leaf size and shape has rarely been investigated and is poorly understood. There are at least six ways in which herbivores, particularly vertebrates and insects, may have influenced the evolution of leaf size and shape, favouring leaf morphologies that differ from those dictated by physiological and biomechanical constraints acting on plants. They are mimicry, not only of leaves of other plant species but also grazed leaves and inanimate objects; crypsis; physical barriers to being eaten; interspecific differences in leaf morphology to reduce recognition by herbivores; very small or highly divided and dissected leaves that reduce feeding efficiency; and different adult and juvenile foliages. There is an urgent need for studies specifically designed to investigate the impact of herbivores on leaf size and shape.