This chapter focuses on the principle of the photoacoustic Doppler (PAD) effect from moving particles, and discusses its potential use in flow sensing. In the PAD effect, the photoacoustic source and the detector move relative to each other while the laser beam is stationary. An important property of the PAD shift is that it is not affected by the possible multiple light scattering from a surrounding medium, which could randomize the laser illumination angles quickly. Very differently, in laser Doppler blood flow measurements, multiple light scattering from biological tissue fundamentally confounds the interpretation of the laser Doppler shift. The conventional Doppler flow measurement methods include both laser Doppler flowmetry and acoustic Doppler flowmetry. They both rely on the scattering-based Doppler effect, in the sense that they both require the presence of small scattering particles to provide Doppler-shifted backscattering signals.