Nickel-titanium rotary instruments have gained considerable popularity among endodontists because the high flexibility of this alloy results in improved clinical efficiency for cleaning and shaping curved root canals, compared to stainless steel instruments. New instrument designs have been introduced recently with variations in rake angle, taper, and machined helixes. Purpose: Use a scanning electron microscope to investigate the patterns of failure in widely used nickel-titanium rotary instrument products and to identify potential factors for instrument failure. Methods: ProSystemGT, ProTaper and ProFile instruments (Dentsply Tulsa Dental), which were obtained with an unknown history of clinical use, had either fractured or experienced considerable permanent torsional deformation. Specimens were cleaned ultrasonically in ethanol prior to SEM examination (JEOL JSM-820), and qualitative energy-dispersive x-ray spectrometric analyses (EDS) were performed to identify surface deposits. Results: SEM observations revealed the following: (a) machining marks with microcracks on instrument surfaces, (b) characteristic metal rollover at flute edges, (c) adherence of dentinal debris under the rollover and wedged into microcracks near the fracture surface, (d) evidence of ductile fracture from regions of dimpled rupture, (e) oxide particles from the manufacturing process, (f) the presence of characteristic striations, (g) pitted features on some fracture surfaces, (h) fractures parallel to the axis of some instruments, which may be related to these pits, and (i) substantial unwinding of flutes for some instruments that had not fractured. Conclusions: The failure process for rotary instruments is complex and appears to have both ductile aspects and brittle character associated with crack propagation from machining damage. Microcracks and perhaps metal rollover are key factors, and lodged dentin chips cause cracks to grow with continued instrument use.