  Cosmic strings provide a radically different paradigm for the formation of
structure to the prevailing inflationary one. They afford some extra technical
complications: for example, the calculation of the power spectrum of matter and
radiation perturbations requires the knowledge of the history of the evolution
of the defects in the form of two-time correlation functions. We describe some
numerical simulations of string networks, designed to measure the two-time
correlations during their evolution.
