Abstract: Both Townsend theory and the traditional streamer model are the basis for study on gas discharge, however, they both have drawbacks dealing with nanosecond pulse gas breakdown. Based on the traditional mechanisms, the paper summarizes nanosecond pulse breakdown mechanisms in gases, presents some analyses on the validity of streamer criterion in the range of nanosecond pulse, describes the phenomenon of electron runaway and the theory of fast ionization wave breakdown, and gives some simulation calculations about runaway process of high-energy fast electrons. The results show that the runaway breakdown model induced by high-energy fast electrons is promising as accounting for the nanosecond pulse discharge in gases.