Carbon catabolite repression (CCR) is the paradigm of cellular regulation. CCR happens when bacteria are exposed to two or more carbon sources and one of them is preferentially utilised (frequently glucose). CCR is often mediated by several mechanisms, which can either affect the synthesis of catabolic enzymes via global or specific regulators or inhibit the uptake of a carbon source and thus the formation of the corresponding inducer. The major CCR mechanisms operative in Enterobacteriaceae and Firmicutes are quite different, but in both types of organisms components of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS) and protein phosphorylation play a major role. PTS-independent CCR mechanisms are operative in several other bacteria.