Postural perturbations are often used in experimental investigations of human balance and postural control. Treating the perturbation as the input and the resulting postural sway response as the output, an input/output model of the posture-control system can be identified. Selection of the perturbation parameters can have a substantial influence on the accuracy of the identification and on the interpretation of the results. Pertinent parameters include type of perturbation, waveform, power spectrum, bandwidth, amplitude and test duration. The selection of these parameters is discussed in the light of general requirements for persistent excitation, accurate identification, stationarity and subject safety. Experimental results are used to illustrate specific issues.