  We give an overview of the work of Corlette, Donaldson, Hitchin and Simpson
leading to the non-abelian Hodge theory correspondence between representations
of the fundamental group of a surface and the moduli space of Higgs bundles. We
then explain how this can be generalized to a correspondence between character
varieties for representations of surface groups in real Lie groups G and the
moduli space of G-Higgs bundles. Finally we survey recent joint work with
Bradlow, Garc\'ia-Prada and Mundet i Riera on the moduli space of maximal
Sp(2n,R)-Higgs bundles.
