  We summarize the properties of Infrared Dark Clouds, massive, dense, and cool
aggregations of interstellar gas and dust that are found througout the Galaxy
in projection against the strong mid-infrared background. We describe their
distribution and give an overview of their physical properties and chemistry.
These objects appear to be the progenitors of high-mass stars and star
clusters, but seem to be largely devoid of star formation, which however is
taking place in localized spots.
