Extraction efficiency, beam quality, misalignment sensitivity and thermal lensing of stable and unstable resonators were investigated, both experimentally and theoretically. The theoretical description was based on a model combining diffraction theory with the amplification of the electric field inside the active medium. Saturation effects on mode vo-lume and losses are discussed. Experiments were performed with a pulsed Nd:YAG system. A detailed comparison of the two kind of resonators, indicates several advantages of the unstable one, such as low misalignment sensitivity and high far-field brightness. Furthermore, several methods of improving the beam quality of unstable resonators were investigated.