Unless there is a statement to the contrary, the numerical results given here apply to masers which (a) employ cobalt-diluted potassium chromi-cyanide as the active material, (b) have a 10% filling factor, (c) employ a 9.4 Gc/s pump field, (d) amplify or oscillate at 2.8 Gc/s and (e) operate at 1.25°K. Cavity maser amplifiers have a constant root-gain/fractional-bandwidth product. In the case of a one-port cavity maser with negligible ohmic losses, this product is equal to 2/(|Qm| + Qs), where Qm is the magnetic Q-factor and Qs is the Q-factor of the signal transition. The orders of magnitude of |Qm| and Qs are 1000 and 50, respectively. The magnitude of |Qm| can be reduced drastically by using a material which can be pumped at frequencies in the millimetre wavebands. The root-gain/fractional-bandwidth product of a two-port cavity maser cannot be greater than one-half of that of a one-port cavity maser which employs the same active material and has the same magnetic and ohmic Q-factors. Travelling-wave masers with negligible ohmic losses have a constant root-log (half-gain)/fractional-bandwidth product which is equal to (log 2)1/2/Qs when the ohmic losses are negligible. The relaxation parameters which control the large-signal characteristics of a maser are the product of the spin-spin and the spin-lattice relaxation times for the signal transition and the product of the Overhauser parameters for the pump and the signal transitions. Appreciable gain saturation occurs, in cavity masers, when the power input is a few millimicrowatts and, in travelling-wave masers, when the power input is a few hundred millimicrowatts. The power output from a cavity maser oscillator is about a microwatt.