The paper reviews the existing multifrequency signals available for linear system identification in the frequency domain. The motivation for an excitation with a specified Fourier amplitude spectrum, yet having a small number of signal levels, is then outlined. A design algorithm is described for the design of multilevel signals realising an arbitrarily defined set of Fourier specifications. It is shown that the properties of the multilevel signals make them a strong candidate for linear system identification, as well as for the detection of nonlinear effects.