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Microsystems using electrothermal bent-beam microactuators have been demonstrated for a variety of applications including optical attenuators, RF switches, and micro positioners, thus creating a need for information on the longevity of these devices. This paper reports on the dc and pulse mode lifetime testing of this class of actuators constructed using polysilicon and p++ doped single crystal silicon. The relative temperature profile along the top surface of an actuator is experimentally verified by scanning probe microscopy. Displacement measurements are used to explore links between aging behavior and the design variables and operating conditions. At low power levels (which result in average operating temperatures of 300-400°C) both polysilicon and p++ Si devices provide continuous dc operation for >1400 min, in air without change in amplitude. While some types of p++ Si devices show monotonic loss of amplitude in pulse tests, others have been operated up to 30 million cycles without degradation. The displacement for polysilicon actuators can either increase or decrease depending on the geometry of the device and operating conditions, both of which are related to temperature and stress of the structural members. Polysilicon grain transformations are observed over extended operation at high temperatures. Performance changes are correlated to material properties using SEM and TEM images.