MEMS (MicroElectroMechanical Systems) sensors are used in acceleration, flow, pressure and force sensing applications on the micro and macro levels. Much research has focused on improving sensor precision, range, reliability, and ease of manufacture and operation. One exciting possibility for improving the capability of micro sensors lies in exploiting the piezoresistive properties of silicon, the material of choice in many MEMS fabrication processes. Piezoresistivity—the change of electrical resistance due to an applied strain—is a valuable material property of silicon due to its potential for high signal output and on-chip and feedback-control possibilities. However, successful design of piezoresistive micro sensors requires a more accurate model of the piezoresistive behavior of polycrystalline silicon. This study sought to improve the existing piezoresistive model by investigating the piezoresistive behavior of compliant polysilicon structures subjected to tensile, bending and combined loads. Experimental characterization data showed that piezoresistive sensitivity is greatest and mostly linear for silicon members subject to tensile stresses and nonlinear for beams in bending and combined stress states. The data also illustrated the failure of existing piezoresistance models to accurately account for bending and combined loads. Two MEMS force and displacement sensors, the integral piezoresistive micro-Force And Displacement Sensor (FADS) and Closed-LOop sensor (CLOO-FADS), were designed and fabricated. Although limited in its piezoresistive sensitivity and out-of-plane stability, the FADS design showed promise of future application in microactuator characterization. Similarly, the CLOO-FADS exhibited possible feedback control capability, but was limited by control circuit complexity and implementation challenges. The piezoresistive behavior exhibited by the Thermomechanical In-plane Microactuator (TIM) led to a focused effort to characterize the TIM's behavior in terms of force, displacement, actuation current and mechanism resistance. The gathered data facilitated the creation of an empirical, temperature-dependent model for the specific TIM. Based on the assumption of a nearly constant temperature for each current level, the model predicted the force and displacement for a given fractional change in resistance. Despite the success of the empirical model for the test TIM device, further investigation revealed the necessity of a calibration method to enable the model's application to other TIM devices.