Abstract: This paper reviews the observational facts and theories on the airflow over small mountains.Special emphasis is laid to point out and discuss the controversies in the theory of lee waves among different writers.According to present author controversiesare due to (1) the difference in scale of the mountain and (2) the choice of the upper boundary condition.When l is comparable to the critical(LS/2π),(LS=u/√(g/θ ∂θ/∂z),where l is the half-widch of the mountain,lee waves occur.When there is an upper solid lid or a discontinuity in the atmolee waves occur.In the first case the forced vibration is in resonance with natural vibration.In the second case there can exist only vibrations of certain wave lengths,which appears as those of the lee wave.The physics of lee waves is thus clear.