  We invistigate exact solutions for the two-dimensional quantum field theories
called Wess-Zumino-Novikov-Witten (WZNW) models. A WZNW model is a sigma model
whose classical fields are applications from a bidimensional space-time (a
Riemann surface in the euclidian case) to a Lie group, the target space. We
construct (and we compute in genus zero and one) the metric connection, called
the Knizhnik-Zamolodchikov-Bernard (KZB) connection, on the bundle of conformal
blocks of the WZNW model. The KZB connection may be viewed as a quantization of
Hitchin integrable systems whose configuration space is the moduli space of
principal holomorphic bundles over a Riemann surface and whose phase space is
the (holomorphic) cotangent bundle to the configuration space. For these
systems, we construct explicitly a complete familly of Hamiltonians in
involution in genus zero, one and two, with (complex) group SL(2) for the last
case. The main result is the self-duality property of the Hitchin system at
genus two, that is the invariance of the Hamiltonians with respect to the
interchange of positions and momenta in the phase space. We finally realize the
(geometric) quantization of the Hitchin systems.
