In designing to avoid fatigue failure in polymers, the design engineer is often faced with a Herculean task. For engineering, aesthetic and economic reasons, polymers are marketed in a multitude of compositional and structural variations. Unfortunately, complete (or even partial) fatigue data is seldom available. The design engineer must thus rely on an understanding of the fatigue mechanisms in order to establish meaningful measures of fatigue resistance. An on-going research program on the fatigue behavior of polymers has been conducted in our Laboratory. Two examples are discussed in this paper.