  In this lecture I review the theory of magnetized accretion discs driving
jets, with a focus on Young Stellar Objects (YSOs). I first introduce
observational and theoretical arguments in favor of the ``disc wind'' paradigm.
There, accretion and ejection are interdependent, requiring therefore to
revisit the standard picture of accretion discs. A simple magnetostatic
approach is shown to already provide some insights of the basic phenomena. The
magnetohydrodynamic equations as well as all usual assumptions are then clearly
listed. The relevant physical mechanisms of steady-state accretion and ejection
from Keplerian discs are explained in a model independent way. The results of
self-similar calculations are shown and critically discussed, for both cold and
warm jet configurations. I finally provide observational predictions and the
physical conditions required in YSOs discs. The necessity of introducing a
magnetospheric interaction between the disc and the protostar is briefly
discussed.
