  In this work the most spectacular phenomena occurring during the formation of
a star are briefly reviewed: accretion through a rotating disc of matter and
outflow through the poles of the new stellar object. Magnetic fields have been
proposed to be principally responsible for the coexistence of these opposed
mechanisms of accretion and outflowing. According to different models, the
magnetic fields are either twisted or stretched by the accretion disc, allowing
the formation of polar channels through which part of the accreting matter can
escape. The high degree of coupling between the physical parameters describing
the young stellar object (e.g. circular velocity of the disc and the central
object, viscosity, strength and freezing of magnetic fields, etc.) makes
detailed understanding of the interaction between accretion discs and outflows
very difficult. The models presented here provide only a partial answer to this
difficult problem.
