The ring cavity gives flexibility in resonator design, such as ability to incorporate long collimated beam sections with large beam diameters in order to extract power from large volumes of laser media. Particular attention is paid to the non–planar ring resonator because of its novel features and importance in recent applications in the ring laser gyroscope and in diode–pumped non–planar solid–state ring lasers. For the ring resonator the beam meets the confining mirror at an oblique angle, the radius of the beam in the plane of the ring and normal to it being different. The ring resonator differs from the linear laser in resonant conditions for the frequencies of the longitudinal and transverse modes. In the ring resonator, the electric field distribution is reproduced after one complete transit. The non-planar ring laser cavity with integral optical diode has been used to generate a single unidirectional mode. The geometry of the non-planar ring is chosen to give a small rotation angle.