  The low-energy limits of models with disorder are frequently described by
sigma models. In two dimensions, most sigma models admit either a
Wess-Zumino-Witten or a theta term. When such a term is present the model can
have a stable critical point with gapless excitations. We describe how such a
critical point appears, in particular in two-dimensional superconductors with
disorder. The presence of such terms is required by the underlying (anomalous)
symmetries of the original electron model. This indicates that the usual
symmetry classes of disordered systems in two dimensions can be further
refined. Conversely, our results also indicate that models previously thought
to be in different universality classes are in fact the same once the
appropriate extra terms are included.
