Symbiotic interactions are those relationships between organisms that permit some species to overcome their physiological limitations by exploiting the capacities of others. This volume presents a modern synthesis of scientific knowledge of symbiosis, from the molecular mechanisms underlying its function to the ecological and evolutionary impact of such associations. With an emphasis on basic principles, the book takes the novel approach that symbiosis is a vehicle by which many organisms have gained access to complex metabolic capabilities. Examples are offered to illustrate this concept, including photosynthetic algae in corals, nitrogen-fixing bacteria in plant roots, and cellulose-degrading micro-organisms in herbivorous mammals. The traditional view of symbioses as mutually beneficial relationships is explicitly abandoned. The book draws together the wide-ranging literature on the topic, providing an integrated introduction that is accessible to undergraduates. The work serves as an excellent text for courses in symbiosis, and as a supplementary resource for students in ecology, evolutionary biology, and parasitology. As an up-to-date review of the field, the book will also be valued by graduate students and researchers.