Geomagnetic storms induce electric fields on the ground, which are the source of Geomagnetically Induced Currents (GICs) in grounded conductors such as transmission power systems. Despite being a phenomenon more often associated with countries located at high latitudes, they are not exclusive to them. Indeed, there are accounts of GICs in low latitude countries such as China, South Africa, Brazil or Spain. In this work we will provide a quick overview of Space Weather and Geomagnetism, indicating the sources of variability of Earth’s magnetic field. Moreover, we will show how to compute the electric fields generated by geomagnetic storms and the GICs that are induced in transmission power grids. We use as an illustrative example the case of the southern Portuguese power network. Our study is made within the framework of MAG-GIC project, which aims to characterize GICs in the whole Portuguese transmission power system network. The calculations are based on observations from the magnetic observatory of Coimbra, for which a short description is given.