Transcranial Doppler (TCD) utilizes the Doppler principle to determine the direction and velocity of blood flow. Most TCDs use long sample volumes in order to improve the signal-to-noise ratio and ease the detection of the basal cerebral arteries. Most TCDs use the fast Fourier transform (FFT) method of spectral analysis which produces the typical visual representation of blood flow velocity. The FFT method of spectral analysis is used in most TCD systems because it allows almost instantaneous detection and display of information in a form which is understandable to most observers. Pulsatility and resistance indices reflect characteristics of the Doppler shift velocity waveforms, and indicate the degree of pulsatility of the waveform. TCD is able to detect two of the major causes of neurological deficits that are abnormalities in blood flow and cerebral embolization. These have made it a valuable practical tool for treating patients in diverse clinical disciplines.