Enzymes activated by monovalent cations are abundantly represented in plants and the animal world. They have evolved to exploit Na+ and K+, readily available in biological environments, as major driving forces for substrate binding and catalysis. Recent progress in the structural biology of such enzymes has answered long standing questions about the molecular mechanism of activation and the origin of monovalent cation selectivity. That enables a simple classification of these functionally diverse enzymes and reveals unanticipated connections with ion transporters.