Multipath resonators are well known as optical delay lines. By many zick-zack rays between two mirrors large optical path lengths can be obtained. This passive system can be converted into a laser oscillator by closed ray paths and a Nd:YAG crystal at one mirror. Optical pumping occurs by directly coupled diodes at the reflection points. Very efficient (more than 40% optical/optical) and compact systems up to 20 W, with the beam propagation factor m2 < 3 were generated. Q-switching and internal frequency doubling were also demonstrated. The fundamental physics of this system including the transverse mode-locking at the degeneration points of the resonator was investigated, experimentally and theoretically.