The ignition of a discharge (breakdown) in a low-pressure gas, of the order of 0.1–10 Torr, in the so-called long tubes, in which the length significantly exceeds their transverse size, is considered. The breakdown mechanism in them differs
significantly from that which occurs in the gaps between large flat electrodes creating the initially uniform electric field. That mechanism is described in all monographs and textbooks on gas electronics, the key factor in it is being electron avalanches (Townsend mechanism). In long tubes, the ignition of a discharge is preceded by the traversing the gap by an ionization wave, a region of an electric field of abnormally high strength. Due to this, the nature of the breakdown acquires a number of features that are not typical of an avalanche breakdown. One of them is that the most important discharge ignition characteristic, the breakdown voltage, is susceptible to relatively weak influences, such as room lighting, with constant or pulsed voltage of mild amplitude. They can either reduce or increase the breakdown voltage by up to hundreds of percent. This chapter describes the results of the relevant experiments and provides a qualitative explanation of the observed effects.