  We first review the phenomenology of viscous liquids and the standard models
used for explaining the non-Arrhenius average relaxation time. Then the focus
is turned to the so-called elastic models, arguing that these models are all
equivalent in the Einstein approximation (where the short-time elastic
properties are all determined by just one effective, temperature-dependent
force constant). We finally discuss the connection between the elastic models
and two well-established research fields of condensed-matter physics: point
defects in crystals and solid-state diffusion.
