This volume provides a comprehensive view on the different sources of the geomagnetic field both in the Earth's interior and from the field's interaction with the terrestrial atmosphere and the solar wind. It combines expertise from various relevant areas of geomagnetic and near Earth space research with the aim to better characterise the state and dynamics of Earth's magnetic field. Advances in the exploitation of geomagnetic observations hold a huge potential not only for an improved quantitative description of the field source but also for a better understanding of the underlying processes and physics. Key is the separation of the field sources in the observations, especially, but not solely, during times of quiet geomagnetic conditions, when the most subtle geomagnetic effects can be identified and become significant. The collected articles are based on the current constellation of ground and space observations, and on state-of-the-art empirical models and physics-based simulations. Thus, it provides an in-depth overview over recent achievements, current limitations and challenges, and future opportunities in the field of geomagnetism and space sciences. Originally published in Space Science Reviews, Volume 206, Issue 1-4, March 2017