  This review aims at providing a retrospective, as well as a description of
the state-of-the-art and future prospects regarding the theoretical and
experimental characterisation of negative-U defects in semiconductors. This is
done by complementing the account with a description of the work that resulted
in some of the most detailed, and yet more complex defect models in
semiconductors. The essential physics underlying the negative-U behaviour is
presented, including electronic correlation, electron-phonon coupling,
disproportionation, defect transition levels and rates. Techniques for the
analysis of the experimental data and modelling are also introduced, namely
defect statistics, kinetics of carrier capture and emission, defect
transformation, configuration coordinate diagrams and other tools. We finally
include a showcase of several works that led to the identification of some of
the most impacting negative-U defects in group-IV and compound semiconductors.
