Aliasing occurs in pulsed wave and color Doppler when blood flow velocity exceeds the Nyquist limit — half the pulse repetition frequency (PRF). When this limit is exceeded, the Doppler system cannot accurately determine velocity and direction; the display wraps around and shows apparent flow in the opposite direction. Aliasing can be corrected by increasing the PRF, shifting the baseline, using a lower frequency transducer, or switching to continuous wave Doppler which has no velocity limit.