Mutualistic interactions between ants and plants involve rewards offered by plants and services performed by ants in a mutually advantageous relationship. The rewards are principally food and/or nest sites, and ants in turn perform a number of services for plants: They disperse and plant seeds; they protect foliage, buds, and reproductive structures from enemies such as herbivores and seed predators; they fertilize plants with essential nutrients; and they may sometimes function as pollinators. In this book Professor Beattie reviews the fascinating natural history of ant-plant interactions, discusses the scientific evidence for the mutualistic nature of these relationships, and reaches some conclusions about the ecological and evolutionary processes that mold them. Mutualisms involving single pairs of species are the exception rather than the rule; usually arrays of ant species interact with arrays of plant species. Variation generated by this complexity results in variation in the function and the effectiveness of the mutualism. The result is that at any given time and place some or all of the interacting species may experience full, intermediate, or episodic benefits, or no benefits at all. This highly dynamic picture is unlikely to be confined to ant-plant mutualisms, but rather may be representative of a host of other kinds of species interactions. This important work is the first broad and thorough treatment of the subject of ant-plant mutualisms. Its natural history, experimental approach, and integration with contemporary evolutionary and ecological literature will appeal to a wide variety of biologists.