We have seen that lateral resolution in conventional optical microscopy is limited by light diffraction. This means that objects smaller than approximately half the wavelength of light (λ/2) cannot be properly resolved. Near-field optical microscopy overcomes the diffraction limit by collecting the near-field light, which does not undergo diffraction, and thus produces optical images with significantly higher lateral resolution, typically in the range λ/10-λ/50 (Betzig and Chichester 1993; Hecht et al. 2000; Hartschuh 2008). As a scanning probe technique, scanning near-field optical microscopy (SNOM) can simultaneously provide topological images of objects. In essence, SNOM can in fact be considered a combined technique that encompasses the features of optical and scanning probe microscopies. According to the nature of the scanning probe, the illumination, and the type of optical contrast, SNOM can be classified into a series of derived subtechniques, as seen in Figure 4.1. Those techniques with spectroscopic capabilities, such as tip-enhanced Raman or infrared microscopy as well as fluorescence SNOM, are treated in detail in other chapters.