In this book a scanning near-field optical microscope (SNOM) based on an apertureless scattering technique is introduced for resolving optical properties of surfaces with lateral resolution reaching 10 nm and better. The construction of the instrument is based on a dynamic mode atomic force microscope (AFM) which is coupled with a sophisticated heterodyne interferometric optical detection system. A laser beam is focused onto the apex of the metallic or dielectric AFM tip. The backscattered light is collected and interfered with a reference beam which is slightly shifted in frequency with respect to the scattered beam. By demodulating the beat signal at higher harmonics of the tip vibration, the far field can be suppressed effectively, leaving only the near-field information of the surface-tip interaction. The amplitude and phase of the near-field belonging to the surface is obtained simultaneously with topography. The apertureless SNOM (a-SNOM) features several advantages over the well-known aperture SNOM: strong background suppression, high sensitivity to material contrast and a resolution limited essentially only by the tip apex size.