The effect of hypervelocity impacts from micrometeoroids and orbital debris (MOD) on spacecraft materials and structures is discussed in terms of observed flight damage and laboratory simulation tests. Methods of estimating the probability of impact as a function of particle size are presented based on particle distributions and orbital altitude. Two types of damage mechanisms are described: initial impact cratering, and debris plumes emanating from the impact site. Shielding systems currently employed are also described. With the return of human exploration of the moon, the effect of meteoroid impacts on lunar structures and equipment is also examined.