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 ofthe "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 into 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 fromdiscs are explained in a model independent way. The results of self-similar calculations are shown and criticallydiscussed, for both cold and warm jet configurations. I finally provide observational predictions and the physical conditions required in YSOs discs. The need for introducing a magnetospheric interaction between the disc and the protostar is briefly discussed.