Abstract: A method for distinguishing between individual human subjects using plethysmographic waveforms. In one embodiment, the method comprises the steps of: receiving a plurality of plethysmographic signals from different individuals; normalizing the signals to the same period length and mean; changing the basis of each of the plurality of signals to obtain its basis vectors and coefficients; reducing the number of coefficients for each signal in the plurality of signals; and calculating the covariance matrix and mean for each individual's set of normalized and dimensionally-reduced plethysmograph waveforms. In another embodiment, the method further comprises the step of taking the Mahalanobis distance between each new plethysmograph waveform that has also been normalized and dimensionally reduced and the stored models of all individuals. In yet another embodiment, the method further comprises the step of finding the index of the stored model with the minimum Mahalanobis distance.