In an 1831 paper, Michael Faraday described observations of the interaction of a vibrating solid with a liquid supported by the solid. The motions induced on the surface of the liquid, which he termed crispations, resulted from the creation of capillary waves, which are still a subject of research. The contributions to the present Faraday Discussion concern chiefly the use of piezoelectric crystals to study the properties of films and of liquids at liquid/solid interfaces. In this Introductory Lecture, the characteristics of piezoelectric acoustic devices used for sensing are reviewed and contrasted. Recent developments in the fabrication of micromechanical structures to make microelectromechanical systems (MEMS) are reviewed. Some applications of this technology to make ultrasonic piezoelectric devices that sense and actuate liquids and gases are described.