This chapter introduces the potential of biogas and ammonia as alternative low- or zero-carbon renewable fuels for internal combustion engines (ICEs) within the context of the global energy transition. Biogas, a renewable fuel derived from organic waste, and ammonia, notable for its high hydrogen content and safe storage, both offer promising pathways for the decarbonization of transportation. However, challenges remain regarding their direct application in ICEs. Ammonia’s low flammability, low laminar flame speed, and high ignition energy limit its combustion performance, while biogas faces issues related to variable composition and energy density.