Micromachining of silicon allows for low-cost fabrication of sensors with integrated circuits. Most of our microphones have a membrane area of 1 mm x 1 mm. Fields of applications are hearing aids and situations where low vibration sensitivity is important, such as in cassette recorders, camcorders and automobiles. In the last decade several transducing principles were examined: Capacitive sensors in 2-chip-design are biased externally or with electret layers of silicon dioxide or double-layers of silicon dioxide and -nitride. These double-layers can also be used as membrane materials due to their stress-compensating properties. Today's research is focussed on 1-chip-sensors. The advantage of these silicon condenser microphones is their high sensitivity, low vibration sensitivity and high service temperature. Piezoelectric and piezoresistive transducers are characterized by their simple design: The former have thin layers of piezoelectric polymer P(VDF-TrFE) on a silicon nitride membrane while the latter use the piezoresistivity of mono- and polycrystalline silicon. Due to the importance of optical data transfer, microphones transducing sound into optical waves are quite attractive for future applications. We work on amplitude and phase modulating sensors. A new field of interest is the realization of micromachined acoustical actuators for hearing aids. In the last few years, new technologies were invented giving the possibility to create micromechnical components. An application one can think of is the use in ITE (In The Ear) hearing aids. This application requires small sized actuators having good acoustical properties.