The field of piezoelectric thin films for micro and nano systems combines an exciting richness of potential applications with many attractive scientific topics on materials processing and physical properties. Piezoelectricity transforms a mechanical stimulus into an electrical signal, or electrical energy. Miniature thin film devices detect and measure vibrations and acoustic waves, as well as generate electrical power in the mW range by the harvesting of vibration energy. An electrical stimulus can be applied to generate acoustic waves, to damp actively vibrations detected by the same film, or to drive a micro robot. The ability to act in both directions of transfer between mechanical and electrical energy allows for high-performing filters, oscillators, and gravimetric sensors working at frequencies up to10 to 20 GHz. While rigid piezoelectric thin films like AlN excel in GHz applications such as RF filters, ferroelectric thin films like Pb(Zr,Ti)O3 are more efficient in energy conversion and include as further dimension a programmable polarity, which is useful for memory applications.