Abstract Combustion ignition by electrical sparks and nonresonant laser sparks is preceded by breakdown, electron heating, relaxation of internal plasma energy, shock wave generation and propagation, and subsonic flows. Measurements and model calculations of these processes (which occur over times of 10 −12 –10 −1 s) in laser sparks show that the minimum ignition energy (MIE) for laser sparks is higher than for electrical sparks, fundamentally because of the higher energy cost of laser spark formation (both breakdown and heating), but also because of more efficient removal of the energy absorbed in laser sparks to regions outside the nominal ignition kernel by the shock. The MIE is bounded below by the breakdown energy, which depends strongly on the focal length of the lens in laser systems; this partly explains the wide range of reported laser spark MIEs. Another possible factor leading to a lower MIE for electrical sparks is supplemental heating of the kernel region long after emission of a shock wave, an effect entirely absent in laser sparks. Energy removal by shocks is more efficient for both types of spark than convective energy transport or diffusion out of an ignition kernel, so the spark ignition MIEs inevitably exceed the thermal MIE based on chemical kinetics for local heating of the kernel region.