This chapter examines the following concepts: damage and its effects, damage prevention, damage assessment, smart structures, and repair. The philosophy of damage to composites has been expressed as: If the damage is visible, it should be repaired or the part should be scrapped. If the damage is not visible, the structure should be capable of withstanding the required residual strength load level. This section looks at how material choice might be altered for parts where damage is either frequent or catastrophically important. In addition to the material-based concepts for improving damage tolerance already discussed, the design of the composite can be optimized for damage tolerance. Because composites are usually brittle materials, impacts often result in damage to the fibers and the matrix. When composites have been optimized for weight and performance, and especially in critical applications, this damage can be catastrophic. Therefore, great efforts have gone into preventing damage or into reducing its effects. Sometimes, without much of a performance penalty, material changes can be made to reduce the effect of the damage. It is also sometimes possible to change the design of the part to reduce damage effects.