An elementary theory is proposed for estimating the strength of pressure waves to be expected from sudden breakdown of a gas by high frequency electromagnetic waves in a one‐dimensional geometry. It is shown that, for an uncontrolled breakdown where the local field strength is not carefully matched to the instantaneous plasma condition to avoid strong reflections, the heating effect will be self limiting and the resultant shock strength depends only on the incident wave frequency and on the initial gas density. A numerical example for microwave breakdown in air indicates that, at normal sea level density, the shock wave accompanying the breakdown is generally quite weak (of the order of Mach 1.1 at 100 Gc/sec frequency). However, for breakdown at lower air densities, stronger shock waves can be anticipated. Preliminary experimental observations of pressure waves generated in a one‐dimensional channel by pulse microwave discharges tended to confirm the general features of the elementary theory but showed somewhat stronger effects than predicted.