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A review is presented of ISO observations of molecular hydrogen and its isotopic species, HD, toward various Galactic source types, such as shocks (Orion, DR 21, Cep A), photon dominated regions (IC 1396A, S140, Orion Bar) and putative X-ray excited sources (RCW 103, SS 433). In so doing I examine the similarities and differences in the H2 spectrum found under these different excitation conditions and mechanisms, and how the observations impact on some of the latest shock and PDR models. For instance, in addition to the ubiquitous ~ 2000 K component in shocks, seen using ground-based instrumentation, ISO reveals the existence of gas with both lower (150–800 K) and higher (≥ 3000 K) excitation temperatures. Further, ISO allows for the first time a measure of the bulk gas temperature toward PDRs (100–700 K), and H2 is also detected where X-ray emission is observed, providing at least circumstantial evidence of X-ray heating.
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