Protective ant-plant interactions, important in both temperate and tropical communities, are increasingly used to study a wide range of phenomena of general interest. As antiherbivore defenses "worn on the outside," they pose fewer barriers to experimentation than do direct (e.g., chemical) plant defenses. This makes them tractable models to study resource allocation to defense and mechanisms regulating it. As multi-trophic level interactions varying in species specificity and impact on fitness of participants, ant-plant-herbivore associations figure prominently in studies of food-web structure and functioning. As horizontally transmitted mutualisms that are vulnerable to parasites and "cheaters," ant-plant symbioses are studied to probe the evolutionary dynamics of interspecies interactions. These symbioses, products of coevolution between plants and insect societies, offer rich material for studying ant social evolution in novel contexts, in settings where colony limits, resource supply, and nest-site availability are all more easily quantifiable than in the ground-nesting ants hitherto used as models. PROTECTIVE ANT-PLANT MUTUALISMS 427 Other plants lack obvious ant-specialized traits but frequently harbor ant-tended hemipteran trophobionts. Recent reviews of ant-hemipteran interactions are provided by Delabie (2001), Gullan (1997), and Gullan & Kosztarab (1997). Most ant-tended hemipterans are phloem-feeders and excrete excess liquid as sugarrich honeydew. An important resource for ants, honeydew-producing hemipterans are often monopolized by territorial, ecologically dominant ants (Blüthgen et al. 2000). These generalist predators can strongly reduce densities of phytophagous insects.