The SAM-SAH riboswitch is a genetic element found in bacterial genomes that controls the expression of genes involved in the biosynthesis and utilization of S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH). This regulatory RNA molecule acts by binding either SAM or SAH and undergoing a structural change that affects the translation or stability of downstream mRNA. Synonyms for the SAM-SAH riboswitch include SAM/SAH riboswitch, AdoMet-SAH riboswitch, S-box leader, and S-adenosylmethionine riboswitch. These various names reflect the different ways in which this RNA element has been studied and characterized by different research groups over the years. Nonetheless, all of these terms refer to the same functional RNA structure that plays a critical role in bacterial metabolism.