Physical processes of microwave backscattering from wind wave surfaces are investigated in a wind wave tunnel by using a X-band (9.6 GHz) microwave scatterometer. Detailed analysis of time series of the backscattered intensity and Doppler spectrum shows that the physical processes of microwave backscattering are closely associated with the processes of the wind wave field. At slant incidence (45¿) the backscattered intensity is in phase with the wave profile, and the Doppler velocity also follows the phase of the individual waves with high Doppler velocity observed at the crests of the individual waves. This velocity is equal to the propagating speed of the crests. It is concluded that the fine structures of wind wave surfaces, which are trapped near the crests and propagating with the crests, are the main contributor to microwave backscattering. It is also pointed out that the effect of wave breaking with bubble entrainment on the microwave backscattering is not significant under the condition of the present experiment. At normal incidence the backscattered intensity has a different phase relationship with the wave profile. The intensity has two peaks, one at the crest and the other at the trough of the individual waves. The peak at the trough is stronger than that at the crest. This result is consistent with specular point scattering and also with the asymmetrical features of young laboratory wind waves. At intermediate incident angles, backscattering processes are rather complicated because both specular point scattering and Bragg resonance scattering contribute to the radar backscattering. ¿ American Geophysical Union 1993