Large scale magnetic fields are ubiquitous in the universe. Observations of galaxies and clusters, even at high redshift, show that they are present in these astrophysical objects with relatively high amplitude, and correlated on the same scale of the object itself. However, the origin of these magnetic fields is still unknown: understanding when and how they formed, and how they evolved after formation constitutes one of the problem of modern cosmology. This seminar presents a review of the subject, focussing in particular on the possibility that the observed magnetic fields are primordial, i.e. created well before the formation of the structures in which they are observed. I will discuss how the presence of primordial magnetic fields can affect the evolution of the universe, and consequently how one can constrain their amplitude and possibly detect their presence.