  The van der Waals liquid orthoterphenyl has long been used as a model system
in the study of the glass transition. Motivated by mode-coupling theory,
extensive experiments have been undertaken to monitor the onset of structural
relaxation on microscopic time and length scales. Using in particular
quasielastic neutron scattering, the decay of density and tagged-particle
correlations has been measured as a function of temperature, pressure and wave
number. A consistent picture is developped in which the mode-coupling
singularity appears as a change of transport mechanism in the moderately
viscous liquid, at temperatures far above the conventional (caloric) glass
transition.
