The electrical breakdown characteristics of long spark gaps have been modified by laser‐induced ionization. The mean velocity of −500‐kV streamers is increased by an order of magnitude, to[inverted lazy s]3×108 cm/sec, and discharge columns are guided over a straight path in air and other atmospheres, by a 1.06 &mgr; optical beam focused to[inverted lazy s]10 11 W/cm2.Time‐resolved studies of the experimental phenomena, together with theoretical models for the evolution of laser‐induced ionization, indicate that a continuous low‐level ionization, rather than the occasional optical breakdown beads along the beam path, is responsible for the observed effects. Analysis of the discharge data and direct measurement of the ionization generated by laser radiation focused to subbreakdown intensities both confirm that positive and negative ion pair densities in the range 1010−1011/cm3are responsible for directing and increasing the streamer velocities; photodetachment of electrons by the luminous streamer tip enhances the streamer propagation. Our model can also be used to interpret previously reported optical influence on electrical breakdown.