It has been a common practice to observe changes in the structural and chemical characteristics of polymers to comprehend their degradation process. The environmental conditions that a polymer is exposed to have a significant impact on its service life. However, the polymer’s stability, including nanocomposites or polymer blends, governs its value. The characteristics of degradation can be investigated by using different appropriate analysis techniques. Therefore, the purpose of this chapter is to discuss the variables that may have a major impact on the degradation and accelerated stability testing of the polymer. This chapter discusses a variety of harsh conditions, such as artificial weathering, hot/wet conditions, and chemical exposure, with a primary focus on polymers and fiber-reinforced plastics (FRP).