We have used the molecular beam growth technique which we call “cleaved edge overgrowth” to fabricate quantum wire lasers, in which 1D quantum confinement is entirely defined by the growth process. The active region of our lasers consists of atomically precise quantum wires that form at the T-shaped intersections of 7 nm wide GaAs quantum wells grown along the [001] crystal axis and after an in situ cleave along the [110] crystal axis. The origin of the quantum mechanical bound state is the relaxation of quantum well confinement at this intersection. The high degree of structural perfection achievable in this way allows the observation of stimulated optical emission from the lowest exciton state in optically as well as in electrically pumped devices. The formation of a linear p-n junction in which the quantum wires are embedded is achieved by doping with Be and Si in the two orthogonal growth directions. Efficient current injection into the wires is demonstrated by the almost complete suppression of optical emission from the quantum well states as well as by threshold currents as low as 0.4 mA for uncoated devices at 1.7 K.