The in-plane thermopower of most of the cuprate superconductors (including the new Hg-based materials) shows a systematic pattern as the hole concentration is increased and conductivity increases, namely an approximately temperature-independent shift to more negative values. This pattern is intimately related to the value of Tc, and so possibly to the mechanism of the superconductivity. In the overdoped limit where Tc is reduced, the thermopower is often approximately proportional to temperature, as for conventional metallic diffusion thermopower, and as seen in fullerene superconductors. A remarkably similar pattern of thermopower shifts (but of smaller magnitude) is seen in NbN films. The similarity of cuprate and non-cuprate thermopower suggests a qualitatively similar mechanism. © 1995.