  Classical T Tauri stars are low mass young forming stars that are surrounded
by a circumstellar accretion disc from which they gain mass. Despite this
accretion and their own contraction that should both lead to their spin up,
these stars seem to conserve instead an almost constant rotational period as
long as the disc is maintained. Several scenarios have been proposed in the
literature in order to explain this puzzling "disc-locking" situation: either
deposition in the disc of the stellar angular momentum by the stellar
magnetosphere or its ejection through winds, providing thereby an explanation
of jets from Young Stellar Objects.
  In this lecture, these various mechanisms will be critically detailed, from
the physics of the star-disc interaction to the launching of self-confined jets
(disc winds, stellar winds, X-winds, conical winds). It will be shown that no
simple model can account alone for the whole bulk of observational data and
that "disc locking" requires a combination of some of them.
