When we consider a medium formed of a large number of small components, a system of closely placed bodies or a medium formed of components with strongly different (contrast) properties, we usually find small or large parameters naturally related to the structures under consideration. Sometimes we found not one but two or even more small or large parameters. For a composite body formed of large number of small components, the natural small parameter is a characteristic dimension of the components (usually, as compared with the dimension of the body). If, in addition, composite material is formed of contrast components, there appears one more parameter — ratio of material characteristics of the components.