Rotating instruments are becoming increasingly significant in the scaling and planing of the root surface. The objective of this in vitro study was to test various root-planing instruments on extracted teeth and then to compare the treated surfaces using scanning electron microscopy. Two manual instruments (scaler and curette) and five mechanically rotating instruments (Desmo-Clean; Perio-Set; Viking-Set; and 40-um and 15-um diamond finishers) were investigated. From a total of 42 teeth, six ro ot surfaces were treated with each instrument. The results confirm the clear superiority of the manual instruments (especially the curette). The manual instruments permit good root planing with minimum ablation from the root surface and only a thin smear layer (a compound of grindign dust, dentinal fluid, and water). The best planing results by rotating instruments were achieved with the Desmo-Clean and the 15-um diamond finisher, whose performance was almost equal to that of the manual instruments. The rotating instruments, however, were associated with higher ablation and a marked smear layer. Manual instruments remain the media of choice on easily accessible root surfaces; however, rotating instruments are of advantage in inaccessible areas (eg, furactions) because of their handling properties.