(1) Stellar magnetic fields. Spectro-polarimetry is a very powerful tool to diagnose magnetic fields in a large variety of astrophysical objects. My research work is mainly focused on the polarisation of spectral lines to study the magnetic properties of stellar atmospheres. The Sun allows us to study in deep detail the properties of the magnetic fields and their interaction with the solar atmosphere. However, the Sun is just a single star and thus, we need from others to really constrain and understand the dynamos that generate those magnetic fields in stellar plasmas. In this talk, I will present a new approach to the analysis of the polarised spectra of solar-type stars as well as a recent work focused on the small-scale emergence of magnetic flux in the quiet solar atmosphere. (2) TBD. (3) TBD.