The study of manganites has been undergoing intensive development, especially following the discovery of colossal magnetoresistance (CMR). The most fundamental property of these materials is a strong correlation between their transport and magnetic properties. A transition to the ferromagnetic (and metallic) state occurs at a finite doping level and represents a special type of transition which should be described in terms of percolation theory. The same applies for the transition at the Curie temperature. As a result of the percolation theory approach, the view of these materials, both above and below the transition point, is that of inhomogeneous media consisting of tiny islands of interweaving sub-phases. These basic ideas have been now verified experimentally by neutron data, X-ray analysis, Mössbauer spectroscopy, heat capacity and magnetization measurements, etc. The phase diagram as a function of doping displays a peculiar electron-hole asymmetry; this asymmetry as well as other features (e.g., the optical properties) can be explained in the framework of a generalized two-band picture. We trace how the ground state evolves with doping and give a self-consistent analysis of various thermodynamic, optical and transport properties of metallic manganites, isotope effect, etc. It is predicted that giant oscillations in the Josephson current of a S-AFM-S junction will occur as a function of weak external magnetic fields. The contact phenomena are also described.