Glucose is the primary fuel for most cells. Because the amount of available glucose can fluctuate wildly, organisms must sense the amount available to them and respond appropriately. Altering gene expression is one of the major effects glucose has on cells. Two different glucose sensing and signal transduction pathways in the yeast S. cerevisiae – one for repression, and one for induction of gene expression – have recently come into focus. What we have learned about these glucose sensing and signaling mechanisms might shed light on how other cells sense and respond to glucose.