The numerical-simulation method is used to study the formation and propagation of strongly ionizing shock waves from the place of breakdown, and the interaction of these waves with subsequent pulses of laser radiation. The influence on the propagation of the breakdown waves of ionization, electron heat conduction, energy transfer from electrons to ions, and emission from the resultant plasma is investigated. The processes involved in the propagation of optical breakdown in gases at normal densities and at densities of the order of 10 mg/cm**3 are compared.