| Abstract
| - It is less than 30 years since molecular hydrogen was first identified in a non-terrestrial source. Since then, many wonderful discoveries concerning cosmic H2 have been made, in observations, experiments, and theory. It has become evident that H2 is found in many astronomical environments and almost all types of astronomical object. Molecular hydrogen had an important role in promoting baryonic structure in the early universe. It is most abundant in interstellar molecular clouds in the Milky Way and other galaxies. These clouds provide the reservoir of mass from which new stars are formed. The impact of stars on their environments often creates conditions in which H2 can be excited and for which H2 emission is an effective tracer. Examples of all these phenomena are given in this article.
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