Piezo films are thin, piezoelectric membranes that can be used both as source or as receiver. They may be adapted to a variety of in situ and laboratory geotechnical testing needs. This paper summarizes fundamental concepts in piezo film technology and the physical properties of this material. The principles behind common utilizations are reviewed, and three geotechnical applications are described: a low-cost field sensor and modified triaxial and oedometer cells to measure low-strain moduli in resonant and pulse mode.