  Most of the visible matter in the Universe is in a plasma state, or more
specifically is composed of ionized or partially ionized gas permeated by
magnetic fields. Thanks to recent advances on the theory and detection of
cosmic magnetic fields there has been a worldwide growing interest in the study
of their role on the formation of astrophysical sources and the structuring of
the Universe. In this lecture, I will briefly review the importance of the
cosmic magnetic fields both from a theoretical and from an observational
perspective, particularly focusing on stellar and compact objects, the
interstellar medium and star formation regions, and on galaxies, clusters of
galaxies, and the primordial Universe.
