Interstellar dust and gas clouds can be very active on timescales from hundreds to millions of years. They are in most cases chemical factories where dust grains cause many complex molecules to form and get ejected from their surfaces. They act as chemical catalysts. These clouds can also be rotating, or in some of their denser parts, collapsing to form new stars and planetary systems. They can also get stirred up and dissolved by new stars that are born and eject high-speed streams of particles back into the interior of the cloud, or ionize the cloud completely by the enormous quantities of ultraviolet light which they produce.