Expressions for the output frequency characteristics of nonlinear systems are derived for both multitone and general inputs to provide a natural extension of the linear relationship Y(j omega) = H(j omega) U(j omega). Algorithms for computing the frequency range produced by arbitrary order nonlinear outputs are also described as an extension to the well known concept in linear systems that the frequency range of the system output is the same as that of the corresponding input. The results are shown to provide important insight into the frequency domain properties of nonlinear systems.