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Several microactuator technologies have been investigated for positioning individual elements in large-scale microelectromechanical systems (MEMS). Electrostatic, magnetostatic, piezoelectric and thermal expansion represent the most common modes of microactuator operation. This research focuses on the design and comparative performance evaluation of asymmetrical electrothermal actuators. The motivation is to present a unified description of the behavior of the electrothermal actuator so that it can be adapted to a variety of microsensor and microactuator applications. This research compares the tip deflection performance of the asymmetrical single- and double-hot arm electrothermal actuator designs. Deflection measurements of both actuator designs as a function of arm length and applied electrical power are presented. As a practical application of the electrothermal actuator, the recent realization of a MEMS microengine is described, and evidence of its bi-directional motion is presented. The electrothermal actuator and microengine designs were accomplished with the MEMSProreg CAD software program, and they were fabricated using the MEMSCAP Integrated Microsystems Multi-User Microelectromechanical Systems (MEMS) Processreg (MUMPs) foundry.