The physical principles are given of laser jet engines, in which the source of energy (``fuel'') is a laser source of electromagnetic energy located outside the flight vehicle being accelerated. An analysis is made of two mechanisms of producing a thrust: one is the evaporation mechanism involving the evaporation of the propulsive mass of the flight vehicle by the incident laser radiation and the other is the ``explosive'' mechanism due to the laser-induced breakdown of air (``explosion'') and excitation of a shock wave which can propel the flight vehicle. The first mechanism can be used in the atmosphere and in space, whereas the second can be used only in the atmosphere (laser air-breathing jet engine). The main characteristics of laser jet engines (the ratio of the laser power to the thrust obtained, specific impulse, and power conversion efficiency) are obtained and some of them are confirmed experimentally. The promising nature of the use of lasers in jet engines of this kind is stressed.