These days, most products are highly reliable which makes it very difficult or even impossible to obtain failure data on such products within a reasonable period of time prior to product release. Degradation tests are one way to overcome this obstacle by collecting degradation data (measurement of degradation) on such products. Based on different measurement processes, degradation tests can be divided into non-destructive and destructive degradation tests. In this chapter, we discuss a number of these two types of degradation models that have been developed in the literature to describe the degradation paths of products. In addition, some applications of degradation models of these two classes are also discussed.