The superposition principle is applied to the case of two linearly polarized waves, and the dependence of the resulting interference pattern on the relative polarization and phase difference is analysed. The complex representation for the electric field is used and the superposition of an arbitrary number of harmonic waves is considered. The limiting cases of coherent and incoherent waves are discussed. The Young’s experiment is described as an example of interference by division of wavefront. Dielectric films are considered to illustrate the interference by division of amplitude. Haidinger fringes, Fizeau fringes, and Newton’s rings are discussed as further examples of application of interference by division of amplitude. When the reflectivity of a dielectric plate becomes significant, it constitutes a resonant cavity and multiple reflections between its faces must be considered. The interference pattern resulting from such multiple reflections is analysed.