The understanding of fast gas breakdown is of relevance for new developments in pulsed power switching, ultra-wide-band radar, and electronic countermeasures. Physical mechanisms for breakdown in the sub-nanosecond regime, which is associated with voltages far higher than voltages for standard conditions, have been explored only recently. New phenomena include non-steady state distribution functions, non-local processes, and runaway electrons. For applications of fast breakdown, general scaling laws, such as statistical and formative time delays, and breakdown voltages as a function of gas, pressure, gap geometry etc., are needed, along with information about the relevant discharge properties. For gap distances from 1 to 10 mm, voltages from 50 to 300 kV, voltage risetimes < 200 ps, and pressures up to one atmosphere in air, and in argon as a model gas with well known cross-sections, the basic breakdown phenomenology is investigated. Voltage and current measurements with resolutions of 25 to 50 ps reveal the temporal characteristics of the discharge. X-ray diagnostics with rough spectral resolution using absorption of different metal foils yields statements about the energy distribution of runaway electrons, which extends to the equivalent charging voltage even for atmospheric pressure. Streak camera pictures with 5 ps resolution show a multi-channel development of the discharge, with an ionization zone limited to a narrow layer in front of the cathode. Simple analytical calculations valid for the high-energy portion of the electrons, and detailed three-dimensional Monte-Carlo simulations allow, together with the experimental results, a detailed quantitative description of the discharge physics and the scaling laws relevant for applications.