In this paper three of interfacial waves at the boundary of two homogeneous liquids are discussed. As the aim of this paper concentrates mainly to clarify the characteristics of two liquids are confined to these in some cases of treatment.
　In chapter 1 the viscous boundary layer of interfacial wave between salt and fresh water is discussed. A very simple expression of the coefficient of viscous attenuation of waves is obtained, and the characteristics of the structure of interfacial boundary layer is discussed. The result shows the possibility of the breakdown of the laminar layer caused by the viscous instability.
　In chapter 2 the properties of interfacial waves of finite amplitude is examined. The perturbation method by the slope of wave profile is used, and an ambiguity in the approximation of the third order is pointed out. Then the possibility of the inviscid breaking of the interfacial wave of the permanent type is discussed. The result shows the impossibility of breaking, when the density of upper and lower layers is comparable.
　In chapter 3 the detailed discussion of the Kelvin-Helmholtz instability is expressed. At the first order approximation the dynamics of energy transfer between the no-perturbed flow and the perturbed wave is examined. It is different mation the used by J. W. Miles (1959) in general. At the second order approximation the properties of wave profiles are discussed. The characteristics of the instability is obtained at the limit when the density of both layers approaches each other boundlessly.