Almost all analytic studies of cosmic strings so far were
concerned with the simplest, Goto-Nambu strings. However, it is
abundantly clear that in order to describe more realistic cosmic
string networks, and in particular phenomena such as
small-scale structure and charge and current
build-up on strings, one must include additional degrees of
freedom along the strings. These are generically called
`elastic' string models.
In this talk, a generalised analytic string evolution model, based on Carter's transonic elastic model, will be presented. This explicitly includes the effects of small-scale structure, through an evolution equation for the `renormalised' string mass per unit length. The evolution of string networks in the context of this model will be discussed, and some of its consequences will explored.