Impact from wind-borne debris and the consequences of a breach in the building envelope during windstorms are design load criteria to consider in addition to other design actions such as wind pressure. Observations from past windstorm events reveal impact damage to buildings caused by structural members. These observations highlight the importance of testing building envelope components for resistance to wind-borne debris impacts. This chapter refers to wind-borne debris as missiles and the impacted wall or roof system as the target. The potential for damage depends on the missile impact load and the resistance provided by the building envelope. Missile impact loading can be analyzed in terms of missile impact velocity, mass of the missile in terms of kinetic energy and momentum, and missile impact area and nose shape.