We discuss a dynamical model of harmonic generation that arises from surfaces that demarcate two different metals or conductors, or the electron cloud that spills outside a simple metal surface and the interior bulk, having different electron densities, for example a noble metal such as gold, and indium tin oxide (ITO). While in general two adjacent materials may contain free and bound charges that determine their respective dielectric constants, the transition region may be characterized by a large discontinuity in the free electron density, , epsilon-near-zero conditions, or multiple, nested plasmonic resonances. For example, , while the free-electron cloud that spills outside a noble metal surface decreases as a function of position from the hard ionic surface. These discrepancies lead to the prediction that the angular dependence of second harmonic generation (SHG) from a simple planar structure is direction-dependent, and highlights the sensitivity of both SHG and third harmonic generation to the makeup of the surface and what surrounds it. Our calculations also suggest that the nonlinear optical analysis of more complicated, hybrid structures, such as metal/oxide nanoantennas or metasurfaces, should always be performed by including effects that are generally overlooked, such as nonlocal effects (viscosity and pressure); the presence of linear and nonlinear quantum tunneling currents in the nano- and sub nano-gaps between the nanoantenna and embedded nanoparticles; linear and nonlinear contributions of bound (inner-core) electrons to the dielectric constant.