Poly (lactide), i.e., poly (lactic acid) (PLA) is a biodegradable polyester produced from renewable resources. Due to its high mechanical performance and very low toxicity, PLA is utilised for biomedical, pharmaceutical, ecological, industrial, and commodity applications. PLA is susceptible to various types of degradation, such as hydrolytic degradation in the human body and the environment, biodegradation and photodegradation in the environment, and thermal degradation during processing. In biomedical and pharmaceutical applications, the hydrolytic degradation mechanism and rate are crucial factors for determining its performance. In contrast, in most of the environmental, industrial, and commodity applications, an accurate adjustment of the degradation rate as accurate as in biomedical and pharmaceutical applications is not required, or degradation during use can cause a reduction in material performance. However, for recycling PLA to its monomers, information on the mechanism and rate of hydrolytic degradation and thermal degradation at elevated temperatures is indispensable. This book deals with the hydrolytic degradation, biodegradation, thermal degradation, and photodegradation of PLA-based materials with different material factors and degradation conditions or environments.