In Chapter 6.1 we briefly review the main instruments used in today’s ground-based geomagnetic observations, focusing on their performances and working principles (from a user’s point of view). Next, the major measurement methods and systems currently in use will be introduced, with a focus on the latest developments in the field. In Chapter 6.2 electromagnetic (EM) methods will be discussed to study the electrical conductivity structure within Earth in a wide depth range and can be measured at the Earth’s surface by magnetometers and telluric electrodes. In Chapter 6.3 a new technique based on differences in instrument responses from ground-based magnetic measurements that extracts the frequency content of the magnetic field with periods ranging from 0.1 to 100 seconds will be discussed. This method enables the study of field line oscillations using the publicly available, worldwide database of geomagnetic observatories.