The bacterial cell wall is composed of peptidoglycan, and its biosynthesis is an established target for antibiotics. Peptidoglycan is assembled from a glycopeptide precursor, Lipid II, that is polymerized by peptidoglycan glycosyltransferases into glycan strands that are subsequently cross-linked to form the mature cell wall. For decades bacteria were thought to contain only one family of enzymes that polymerize Lipid II, but recently, the ubiquitous Shape, Elongation, Division, and Sporulation (SEDS)-family proteins RodA and FtsW were shown to be peptidoglycan polymerases. Because RodA and FtsW are essential in nearly all bacteria, these enzymes are promising targets for new antibiotics. However, almost nothing is known about the mechanisms of these polymerases. Here, we report that SEDS proteins synthesize peptidoglycan by adding new Lipid II monomers to the reducing end of the growing glycan chain. Using substrates that can only react at the reducing end, we also show that the glycosyl donor and acceptor in the polymerization reaction have distinct lipid requirements. These findings provide the first fundamental insights into the mechanism of SEDS-family polymerases and lay the groundwork for future biochemical and structural studies.