  Cosmic strings are one-dimensional topological defects which could have been
formed in the early stages of our Universe. They triggered a lot of interest,
mainly for their cosmological implications: they could offer an alternative to
inflation for the generation of density perturbations. It was shown however
that cosmic strings lead to inconsistencies with the measurements of the cosmic
microwave background temperature anisotropies. The picture is changed recently.
It was shown that, on the one hand, cosmic strings can be generically formed in
the framework of supersymmetric grand unified theories and that, on the other
hand, cosmic superstrings could play the r\^ole of cosmic strings. There is
also some possible observational support. All this lead to a revival of cosmic
strings research and this is the topic of my lecture.
