The increasing density of data transmission, speed of all-optical signal processing, and demand for higher resolution microscopy and spectroscopy stimulate the development of the nanophotonics. Near- field microscopy is not limited by light diffraction and thus it can achieve sufficiently subwavelength resolution. Therefore this approach is perfect for nanophotonic device characterization. Heterodyne detection allows resolution of the optical phase and improves signal-to-noise performance in near-field microscopy. The book describes a Heterodyne Near- field Scanning Optical Microscope (HNSOM) and applications of this approach to characterization of several classes of the photonic nanodevices.