This chapter describes an approach that depends on the nonlinear nature of the interaction between tip and sample; this has become known as ultrasonic force microscopy (UFM). The combination of acoustic excitation with scanning probe microscopy makes it possible to image and study the elastic and viscoelastic properties of materials with nanoscale spatial resolution. For the applications described in the chapter, the key components of the UFM and the mechanical diode principle are: the inertial stiffness of the cantilever at the ultrasonic vibration frequency; nonlinear detection of additional forces at low frequency and the compliance of the cantilever at the detection frequency. The shape of the force versus indentation curve depends on surface adhesive and elastic properties. In addition to the elastic properties that UFM is intended to image, anything else that affects the tip‐surface interaction will also affect the UFM contrast.