  In this m\'emoire we study quasiperiodic cocycles in semi-simple compact Lie
groups. For the greatest part of our study, we will focus ourselves to
one-frequency cocyles. We will prove that $C^{\infty}$ reducible cocycles are
dense in the $C^{\infty}$ topology, for a full measure set of frequencies.
Moreover, we will show that every cocycle (or an appropriate iterate of it, if
homotopy appears as an obstruction) is almost torus-reducible (i.e. can be
conjugated arbitrarily close to cocycles taking values in an abelian subgroup
of G). In the course of the proof we will firstly define two invariants of the
dynamics, which we will call energy and degree and which give a preliminary
distinction between (almost-)reducible and non-reducible cocycles. We will then
take up the proof of the density theorem. We will show that an algorithm of
renormalization converges to perturbations of simple models, indexed by the
degree. Finally, we will analyze these perturbations using methods inspired by
K.A.M. theory.
