  The cosmic string theory for cosmological fluctuations is a good example of
healthy scientific progress in cosmology. It is a well defined physically
motivated model that has been tested by cosmological observations and has been
ruled out as a primary source of primordial fluctuations. Until about fifteen
years ago, the cosmic string theory of cosmological perturbations provided one
of the two physically motivated candidate theories for the generation of
primordial perturbations. The cosmological data that appeared during the last
decade have been compared with the well defined predictions of the theory and
have ruled out cosmic strings as a primary source of primordial cosmological
perturbations. Since cosmic strings are predicted to form after inflation in a
wide range of microphysical theories including (supersymmetric and fundamental
string theories) their observational bounds may serve a source of serious
constraints for these theories. This is a pedagogical review of the historical
development, the main predictions of the cosmic string theory and the
constraints that have been imposed on it by cosmological observations. Recent
lensing events that could be attributed to lighter cosmic strings are also
discussed.
