Stimuli-sensitive change their volume (equilibrium degree of swelling), mechanical properties (elasticity, stiffness) and molecular transport properties in response to a small change in the properties of the swelling agent, like temperature, solvent composition, pH value, ion concentration, etc. Widespread used smart gels take advantages of the volume phase transition induced by a change of temperature. The temperature of volume phase transition depends on the interaction between gel and solvent. For a gel with defined chemical structure it can be changed by the composition of the swelling agent, e.g. content of salt or organic components. For application, e.g. in MEMS, an easy and controlled stimulation of volume phase transition is required. The degree of swelling (Q) and therefore the dimension of gel structures are determined by temperature. It is possible to regulate Q to a predetermined value by heating/cooling. Thermal energy inside a gel-based device can be easily generated and regulated by incorporated heating resistors and temperature sensors. Different structures (micro-spheres, pads, patterned layers) of smart hydrogels are applied. The switching between two different states of swelling is induced by changes of temperature or by changing the environment. Using the example of gel-based microvalves, sensors, sensor arrays, pumps, and chemostats (concentration control of chemical substances) the sensor-actuator properties and advantages of this group of polymers are discussed.