Geophysical methods have become a widely used tool in glaciology and other fields of cryospheric research. Their strength lies in the ability to provide information about the sub-surface without the need to access it directly. In this sense, geophysical methods can be considered a subgroup of remote-sensing techniques. They are relatively fast applications to carry out in the field and they allow the possibility of obtaining information quickly about the spatial distribution of diverse physical properties. This chapter introduces the basic principles of the main geophysical methods, how they work and how they are applied in glaciology. Passive methods are those which measure naturally occurring fields: gravimetry measures variations in the Earth’s gravity field; magnetics measures the Earth’s magnetic field; and seismology detects seismic waves of natural origin. In contrast, active methods employ some sort of controlled source: these include radar, an acronym (radio detection and ranging) referring to the propagation and reflection of electromagnetic (radio) waves; seismics, which refers to active source seismology; geoelectrics, which determines resistivity distribution; and electromagnetic induction, which measures induced currents. In this chapter we focus on active seismic and radar methods, which are the most widely used geophysical methods in glaciology. Active geoelectrics and eminduction are widely applied in periglacial and sea-ice research, respectively, but are not considered.