Sensors of strain-related mechanical quantities relying on the piezoresistive properties of thick-film resistors are being increasingly employed in various measuring and control systems; the purpose of this section is to explain how and why thick-film strain gauges differ from metal and semiconductor strain gauges widely employed in similar devices, and to remark the specific methods and materials enabling the design of variety of new classes of sensors for pressure, force weight and acceleration. The potentials of this technology are are illustred by many examples. It is clear that this class of sensors has not reached yet his full potentials; on one side performance of piezoresistors might be improved with satisfactory models of the physical behaviour of thick film resistors, on the other hand we see a continuous search of new materials and structure for increangly integrated and small sensors. However piezoresistive sensors are the field where the possibility of a dynamic evolution of thick film technology as an advanced sensor technology has been demonstrated by a variety of new sensors developed in a short lapse of time.