  ISO has provided us with a new perspective on gas temperatures in
photodissociation regions through measurements of line emission from
rotationally-excited levels of the H_2 molecule. The H_2 rotational level
populations provide a thermal probe, showing that gas temperatures T_gas approx
500-1000 K prevail in a portion of the PDR where significant quantities of H_2
are present. Such high gas temperatures were unexpected. Theoretical models for
the S140 PDR are presented. Possible mechanisms for heating the gas to such
high temperatures are discussed.
