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This chapter summarizes current understanding on the protective function of polyhydroxyalkanoates (PHA) against various stress factors. In their natural habitats, bacteria are constantly challenged by the environment since they are exposed to rapidly changing conditions such as fluctuations in temperature, pH or salinity and availability of nutrients or radiation. Therefore, to cope with such challenging conditions, bacteria developed numerous strategies which help them to face various stressors. Stress response can be considered as one of the key features determining the success or failure in the process of natural selection in highly competitive environments. The biological role of PHA is complex, since it was observed that the presence of PHA granules in bacterial cells enhances their resistance against numerous stress factors such as high or low temperature, repeated freezing and thawing, osmotic and oxidative pressure or UV-irradiations. There are numerous reports indicating that the presence of PHA granules enhances the stress robustness of bacteria against oxidative pressure.