The phase theory, a general and exact method of investigating the disordered systems which has been developed in recent years, is outlined. It allows us to discuss various aspects of the spectral properties of disordered system from a unified point of view, without the ambiguities such as those often involved in approximate theories. After a systematic formulation of the basic principles of the theory is given, it is applied to the problems of impurity modes associated with isolated impurities, spectral gaps of mixed system, fine structures of the spectrum of disordered system, and the nature of eigenmodes of disordered chain. It is seen that the theory works well for all these problems, and can not only explain or predict several characteristic spectral properties such as peak structures and special points in the spectrum and the localization of eigenfunctions, but also contains the existing specific theories as its special cases.