Dramatic changes in the environment, including habitat degradation and climate change, have focused attention on how individuals and populations respond to a shifting biotic and abiotic landscape. A critical step toward meeting this goal is a clear understanding of the capacity of individuals to defend themselves against threats. Changes in water quality and temperature have direct and indirect effects on fishes. Defensive responses can occur at many levels, from cellular to behavioral actions. The authors in this volume have attempted to provide a general view of the current state of knowledge of fish defenses with respect to pathogens, parasites, and predators, and to point out gaps where further study is needed.