Proteins that bind and hydrolyse GTP are being discovered at a rapidly increasing rate. Each of these many GTPases acts as a molecular switch whose 'on' and 'off states are triggered by binding and hydrolysis of GTP. Conserved structure and mechanism in myriad versions of the switch—in bacteria, yeast, flies and vertebrates—suggest that all derive from a single primordial protein, repeatedly modified in the course of evolution to perform a dazzling variety of functions.