  Ion conduction in noncrystals (glasses, polymers, etc) has a number of
properties in common. In fact, from a purely phenomenological point of view,
these properties are even more widely observed: ion conduction behaves much
like electronic conduction in disordered materials (e.g., amorphous
semiconductors). These universalities are subject of much current interest, for
instance interpreted in the context of simple hopping models. In the present
paper we first discuss the temperature dependence of the dc conductivity in
hopping models and the importance of the percolation phenomenon. Next, the
experimental (quasi)universality of the ac conductivity is discussed. It is
shown the random barrier model is able to reproduce the experimental finding
that the response obeys time-temperature superposition, while at the same time
a broad range of activation energies is involved in the conduction process.
Again, percolation is the key to understanding what is going on. Finally, some
open problems in the field are listed.
