The recently discovered layered copper oxide high-Tc, superconductors are discussed as models of a layered electron gas with highly anisotropic collective modes (layer plasmons). Our view that a combined phonon-plasmon pairing interaction is the mechanism for the high transition temperatures is presented together with the appropriate dielectric function describing the electron collective response. The implications of this model for the interpretation of optical (infrared and optical absorption, reflectivity, and Raman scattering) and related spectroscopies, such as electron and positron loss, are given. In particular, temperature-dependent loss features are shown to arise from the lowest branch of the layer plasmons. © 1989, Optical Society of America.