In most optical materials, the atomic or molecular structure is so fine that the propagation of light within them may be characterized by their refractive indices. When an object has structure which is larger than the wavelength of light, its influence on the propagation of light may be described by the laws of diffraction, refraction, and reflection. Between these two extremes is a region in which there is structure that is too fine to give rise to diffraction in the usual sense but is too coarse for the medium to be considered as homogenous. For this, a full description can only be achieved through a rigorous solution of Maxwell's electromagnetic equations, and resonance phenomena are often observed. Recent developments in microlithography have extended the possibility of generating subwavelength structures, and it is now possible to produce materials with remarkable new optical properties.