  Photodissociation regions (PDRs) and shocks give rise to conspicuous emission
from rotationally and vibrationally excited molecular hydrogen. This line
emission has now been studied with ISO and from the ground in great detail. A
remarkable discovery has been that toward the Orion outflow and other
shock-excited regions, the H_2 level populations show a very high excitation
component. We suggest that these high-excitation populations may arise from
non-thermal pumping processes, such as H_2 formation and high-velocity
ion-molecule collision in partially dissociative shocks. In PDRs such as NGC
7023 however, formation pumping is always less important than fluorescent
pumping. We furthermore present two HD emission line detections toward Orion
Peak 1. This enables the first comparison of the H_2 and the HD excitation,
which surprisingly turn out to be identical.
