Field studies of volcanotectonic structures offer a way of understanding the processes that take place inside volcanoes before eruptions. Collapse calderas and some other large-scale structures are treated separately (Chapter 5), and here the focus is on sheet intrusions, sills, inclined (cone) sheets, and, in particular, dikes. Since they supply magma to most eruptions, it is important to make detailed and accurate observations and measurements of sheet intrusions in eroded sections of active and inactive (extinct) volcanoes. All the techniques described here apply equally well to inclined sheets, so that the term ‘dike’ in the present context also includes inclined sheets. Most of the techniques also apply to sills; the special aspects of field studies of sills are discussed at the end of the chapter. The observations and measurements provide a better understanding of how dikes propagate, the field conditions that encourage dike arrest, as well as the conditions that encourage their propagation to the surface to feed volcanic eruptions. The field data, when combined with geodetic and seismic monitoring data, can be used to test analytical, analogue, and numerical models on internal processes in volcanoes.