A general review of the laser resonator, characterized by its g parameters, is presented. The stable configurations with one and two active media are considered. The laser rod is considered as a thick lens, with refractive power that depends on the pumping level. For the one-rod resonator, fundamental mode operation can be performed in two stability zones. These zones are symmetrical and have the same width as a function of the refractive power of the rod. For a resonator with two rods inside, equal and different refractive powers of the rod at the same pump level are considered. By means of the g parameters and the resonator matrix method, beam characteristics such as the spot size on the mirrors, divergence, beam waists and the positions of the beam waists, and the fundamental mode volume into the rod are described. To obtain fundamental mode operation, plane-plane and convex-concave resonators with one and two Nd:YAG active media are experimentally investigated. The results for energy, beam quality, and divergence are presented.