This chapter investigates the influence of dielectric interface on the boundary conditions of thin metallic layer structures. First, the uniqueness of the dielectric constant of zero-thickness effective uniform medium is examined, and further approximate solutions were derived by applying the two Babinet principles. Further, based on the definition of relative permittivity, an equivalent dielectric model is proposed to replace dielectric interfaces with small thicknesses. When the thicknesses on both sides of the dielectric interface synchronously approach zero, the derived equivalent permittivity degenerates into the previously established effective dielectric constant. Finally, this chapter derives the methodology for transforming the boundary conditions of metasurfaces at a dielectric interface into that in an any uniform medium. This transformation simplifies the theoretical analysis of metasurfaces, providing an efficient framework for subsequent research.