Monitoring ground surface deformation is an essential component of volcano monitoring, along with other techniques, such as seismology, geochemistry and geology. This chapter presents the different sources at the origin of the deformation observed in a volcanic context, followed by a review of the various techniques used for measuring such deformation. It next reviews the conditions for deformation detection with these different techniques. Volcanoes are places of more or less developed hydrothermal activity, depending on sites. Pre-eruptive deformations have spatial extensions and characteristic time scales which are different from a volcano to another and from an eruption to another. Detectability depends on the adequacy between the observed phenomena and the measurement technique. Finally, the chapter presents the most widespread deformation models for understanding measurements, as well as their contribution to monitoring.