We describe an enhanced scattering diagnostic method based on the scattering of an extraordinary electromagnetic wave close to the upper hybrid resonance. The method has a high sensitivity and spatial resolution and allows observation of the plasma density fluctuations with characteristic scale-lengths much smaller than the wavelength of the probing radiation in vacuum. The theory of enhanced scattering is presented together with experimental results obtained in small laboratory installations and in the research tokamak FT-2. The experiments were aimed at the study of small-scale intrinsic fluctuations in quiescent plasmas, the propagation of lower hybrid waves and the parametric instabilities.