  We explore progress in understanding the behaviour of cation conducting
glasses, within the context of an evolving ''dynamic structure model'' (DSM).
This behaviour includes: in single cation glasses a strong dependence of ion
mobility on concentration, and in mixed cation glasses a range of anomalies
known collectively as the mixed alkali effect. We argue that this rich
phenomenology arises from the emergence during cooling of a well-defined
structure in glass melts resulting from the interplay of chemical interactions
and thermally driven ionic motions. The new DSM proposes the existence of a new
site relaxation process, involving the shrinkage of empty $\bar A$ sites (thus
tailored to the needs of $A^+$ ions), and the concurrent emergence of empty
$C'$&#146;sites, which interrupt the conduction pathways. This reduction of
$\bar A$ sites is responsible in the molten glass for the sharp fall in
conductivity as temperature drops towards $T_g$. The $C'$ sites play an
important role also in the mixed alkali effect, especially in regard to the
pronounced asymmetries in diffusion behaviour of dissimilar cations.
