This research focuses on the design and experimental characterization of two types of MEMS asymmetrical electrothermal microactuators. The motivation is to present a description of the behavior of the electrothermal microactuator to facilitate its adaptation to a variety of MEMS applications. Both MEMS polysilicon electrothermal microactuator design variants use resistive (Joule) heating to generate thermal expansion and movement. In a conventional electrothermal microactuator, the 'hot' arm is positioned parallel to a 'cold' arm, but since the 'hot' arm is narrower than the 'cold' arm, the electrical resistance of the 'hot' arm is larger. When an electrical current passes through the microactuator (through the series connected electrical resistance of the 'hot' and 'cold' arms), the 'hot' arm is heated to a higher temperature than the 'cold' arm. This temperature increase causes the 'hot' arm to expand along its length, thus forcing the tip of the device to rotate about a mechanical flexure element. A new electrothermal actuator design eliminates the parasitic electrical resistance of the 'cold' arm by incorporating an additional 'hot' arm. The second 'hot' arm results in an improvement in electrical efficiency by providing an active return current path. Additionally, the 'cold' arm can now have a narrower flexure compared with the conventional single-'hot' arm device because it does not have to pass an electric current. A narrower flexure element manifests improved mechanical efficiency. Deflection and force measurements of both electrothermal actuators as a function of applied electrical power are presented. Also described is the practical integration of the electrothermal microactuators in a monolithic microengine that is capable of rotating a set of gears.