  Cosmic strings are predicted by many field-theory models, and may have been
formed at a symmetry-breaking transition early in the history of the universe,
such as that associated with grand unification. They could have important
cosmological effects. Scenarios suggested by fundamental string theory or
M-theory, in particular the popular idea of brane inflation, also strongly
suggest the appearance of similar structures. Here we review the reasons for
postulating the existence of cosmic strings or superstrings, the various
possible ways in which they might be detected observationally, and the special
features that might discriminate between ordinary cosmic strings and
superstrings.
