Ionospheric small-scale irregularities are commonly detected with radar systems in both E and F regions from subauroral latitudes to the polar cap. In the E region, numerous observations performed at frequencies from HF to UHF have led to an understanding of the main features of the generating plasma instabilities. On the other hand, F region irregularities can only be detected with HF radars due to the aspect sensitivity of the echoes and the ionospheric refraction at these frequencies. Doppler studies have been performed only recently but have already shown that several well-known plasma instability mechanisms can explain the observations. We review in this work the radar observations of these various natural irregularities with emphasis on the recent results and their implications for the theories of their generating mechanisms.