Biotic agents affect millions of hectares of forests throughout the world each year. Of all the environmental stresses affecting trees, insects and diseases are by far the most widespread and most studied but, perhaps, the least understood. Their influence on forest productivity may be relatively small, as during low-level infestations that produce nagging drains on growth, or catastrophic, as during epidemic outbreaks that may destroy thousands of hectares in a short period of time. In contrast to air pollutants, whose effects are primarily regional in nature, biotic pests permeate nearly every forest in all parts of the world and constantly pose a threat to the health of these ecosystems. Biotic pests commonly interact with other agents of stress, both natural and anthropogenic, to produce complex problems for forest managers throughout the world. These complex interactions between biotic pests and other agents of stress are beyond the scope of this chapter and are discussed elsewhere in this book (Chapter 7). This chapter addresses the relationships between susceptible hosts and the major biotic pests in the southern United States.