  A relativistic generalisation of a well-known method for approximating the
dynamics of topological defects in condensed matter is constructed, and applied
to the evolution of domain walls in a cosmological context. It is shown that
there are self-similar ``scaling'' solutions, for which one can in principle
calculate many quantities of interest without recourse to numerical
simulations. Here, the area density in the scaling regime is calculated in
various backgrounds. Remarkably good agreement with numerical simulations is
obtained.
