A chemical mechanical polishing apparatus has a polishing pad, a carrier to hold a substrate against a first side of the polishing surface, and a motor coupled to at least one of the polishing pad and carrier head for generating relative motion therebetween. An eddy current monitoring system is positioned to generate an alternating magnetic field in proximity to the substrate, an optical monitoring system generates a light beam and detects reflections of the light beam from the substrate, and a controller receives signals from the eddy current monitoring system and the optical monitoring system.