This review presents an overview of the state of the art in understanding of the chemistries behind polyethylene terephthalate (PET) degradation under hydrolytic and photolytic conditions, which are the main degradation processes that operate in the natural environment. Laboratory studies, carried out under so-called accelerated weathering conditions, have been reviewed and are compared to findings obtained from field studies through hydrolysis and photolysis from natural exposure. The review concludes by correlating the techniques used to date and our current understanding of the mechanisms and key intermediates involved in PET degradation.