In recent years, there have been intense efforts in the study of the physical principles and applications of the scanning near-field optical microscope. Extensive theoretical analyses, numerical simulations, and experimental investigations have been conducted. The results demonstrate that image resolution and contrast depends not only on the aperture size of the probe and the reflection/transmission of the sample, but also on other parameters and experimental conditions. In this paper, the influences of the operating mode, probe-sample interaction, polarization of light, and detector orientation are discussed. Furthermore, the progress on the use of linear systems transfer function for the characterization of image resolution is reviewed. Finally, future directions in research and development are discussed.