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Experiments show that conventional solid-state lasers can go into oscillation simultaneously in many modes. This is somewhat surprising since it appears impossible to ``eat holes'' in temperature-broadened lines and thus only one or, at most, a few modes should be able to oscillate. However, the spatial variation in the field intensity of the various modes produces nonuniform distributions in the inverted population and one can show that there is little tendency for these distributions to smooth out due to spatial cross relaxation. Such nonuniform distributions could lead to simultaneous oscillation in many modes. Formulas which relate the number of unstable modes to the pump power and various other maser parameters are obtained. The results show that it is exceedingly difficult to obtain single mode operation in conventional masers at high pumping levels. Ways to avoid a nonuniform distribution density and methods to achieve high-power single-mode operation in practice are discussed. It is also possible to show the effect of slow spatial cross relaxation on the spiking behavior.

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