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As a result of the inherent nonlinearity of the beam-circuit wave interaction process in high-power microwave amplifiers, there are harmonics generated when one signal is applied to the input and both harmonics and intermodulation products are generated when multiple component signals are applied. The effect of these generated signals is to reduce the available power levels of the fundamental signals. In this investigation multiple signals, noise, and combinations of these are considered as inputs to high-power traveling-wave and crossed-field amplifiers. A one-dimensional nonlinear theory is developed to consider multisignal inputs and harmonic generation and includes the effects of RF space charge, distributed circuit loss, and the variation of tube parameters with frequency. The theoretical results for the case of two input signals reveal both a strong larger amplitude capturing effect as well as a pronounced lower frequency signal preference. Both of these effects have also been observed experimentally using a high-power continuous-wave octave bandwidth traveling-wave amplifier. In addition to the multisignal studies a band of noise and combinations of noise and single-frequency signals have been applied to the experimental device. Measurements are made on the formation of noise intermodulation sidebands and on the degradation of a single-frequency input signal when a band of noise is simultaneously applied to the input of the traveling-wave amplifier. The noise is noticeably more effective in reducing the signal power level when it is positioned lower rather than higher in frequency with respect to the single-frequency signal. Experimental studies on a high-power continuous-wave wideband crossed-field amplifier are presented for comparison purposes. While the two devices are very similar in some respects they differ in others. For example the crossed-field amplifier shows a preference for a higher frequency signal which is just opposite to the traveling-wave tube results. In general the agreement between the theoretical and experimental multisignal results is good especially when the effects of distributed circuit loss and space-charge forces are included in the non-linear multisignal interaction equations.