Hewlett-Packard Labs is attempting to catapult the memristor, the fourth passive circuit element after resistors, capacitors and inductors, into the electronics mainstream. Invented in 1971, this "memory resistor" represents a potential revolution in electronic-circuit theory akin to the invention of the transistor—and perhaps its time has finally come. But as with that earlier device, it will take a killer application to get it off the ground.