The cellular concentrations of certain metabolites are assiduously monitored to achieve appropriate levels of gene expression. Although proteins have long been known to act as sensors in this capacity, metabolite-binding RNAs, or riboswitches, also play an important role. More than 20 distinct classes of riboswitches have been identified to date, and insights to the molecular recognition strategies of a significant subset of these have been provided by detailed structural studies. This diverse set of metabolite-sensing RNAs is found to exploit a variety of distinct mechanisms to regulate genes that are fundamental to metabolism.