The wealth of optical phenomena offers interesting opportunities in the world of nanometer dimensions. The field of radiation has to be confined and controlled for this purpose by material structures such as small apertures or slits. Many optical effects manifest themselves in a novel way if the relevant dimensions are small compared to the wavelength. Our theoretical understanding therefore lags somewhat behind the experimental results which have made rapid progress in the past two to three years. This paper concentrates for this reason on the description of a model scanning near-field optical microscope which allows us to gain some insight into the complex problems of coupling light into a nanometer-sized structure, the distribution and possible field enhancement inside such structures, and the radiation from the nanoscale structure into the macroscopic world, where it can be detected by conventional means.