  We have implemented highly stable and tunable frequency references using
optical high finesse cavities which incorporate a piezo actuator. As piezo
material we used ceramic PZT, crystalline quartz, or PZN-PT single crystals.
Lasers locked to these cavities show a relative frequency stability better than
1 x 10^{-14}, which is most likely not limited by the piezo actuators. The
piezo cavities can be electrically tuned over more than one free spectral range
(> 1.5 GHz) with only a minor decrease in frequency stability. Furthermore, we
present a novel cavity design, where the piezo actuator is prestressed between
the cavity spacer components. This design features a hermetically sealable
intra cavity volume suitable for, e.g., cavity enhanced spectroscopy.
