In a process of selectively removing material from an exposed layer carried by a substrate, a technique for determining endpoint by monitoring the intensity of a radiation beam that is passed through the substrate and any intervening layers to be reflected off the layer being processed. This monitoring technique is used during photoresist developing, wet etching, and mechanical planarization and polishing during the manufacture of integrated circuits on semiconductor wafers, flat panel displays on glass substrates, and similar articles. Planarization and polishing processes are alternatively monitored by monitoring temperature.