Microbial electrolysis cells (MECs) represent a renewable hydrogen production technology that offers the possibility of converting wastewater to hydrogen through a bioelectrochemical process. Particularly, the MEC substrate has a significant effect on the performance of MECs, and in this review, the performances of over 30 substrates examined since 2015 are summarized and compared. It was evident that popular MEC substrates include dark fermentation effluents, pyrolysis products, and raw wastewaters. Additionally, the different MEC substrates investigated yielded different MEC performances, indicating that further studies are required before MECs can become a mature technology for up-scale applications.