To face the ongoing issues related to global warming, a circular economy approach should be pursued, rethinking the waste management system and the recovery of organic waste. The main organic waste streams are Food Waste (FW) and municipal Sewage Sludge (SS). In the spirit of circularity, a commingled treatment of FW and SS could be a viable solution. To this end, the present work aims to review the technical and environmental aspects of the co-treatment of FW and SS through biological and thermal processes. Firstly, a detailed characterization of the two substrates is presented as well as the current and future treatment technologies. Then, the technical feasibility and the environmental impacts of conventional biological co-treatments of FW and SS (i.e., composting, anaerobic digestion, and a combination of them), as well as innovative thermal ones (i.e., incineration, gasification, pyrolysis, and hydrothermal carbonization), is summarized. The outcomes of this work could contribute to achieving a more sustainable way to approach organic waste treatment and to help policy-making authorities move toward sustainable planning.