Microbial activity has the potential to change the physical and (geo)chemical conditions of an environment. In a geological repository for nuclear waste, this could have an impact on radionuclide mobility. Intermediate-level radioactive waste that requires geological disposal typically contains a variety of organic compounds, which are prone to degradation. The organic compounds themselves can be prone to microbial degradation. More importantly, chemical and radiolytical degradation processes will result in short-chain organic degradation products and the formation of hydrogen. These can in turn be used as electron donors to stimulate microbial activity with electron acceptors present in the pore waters of host formations or in the waste. This chapter gives an overview of chemical, radiolytical, and microbial degradation processes of cellulose, PVC, ion exchange resins, and bitumen, important organics present in European radioactive waste inventories.