There are many energy sources for the ISM but only a few are likely to trigger star formation. The essential condition is that the energy source has to change a cloud from stable to unstable. Usually this requires some kind of compression, the compressed mass has to exceed an unstable mass, and the compressive force has to last for a time comparable to the collapse time in the compressed region. As mentioned above, individual supernova seem too short-lived to trigger star formation in the ambient ISM, even though they play an important role in energizing the ISM. More important are the HII regions, stellar winds bubbles, and multiple supernovae that occur in OB associations and star complexes. Another long-range and long-lived source of compression is a spiral density wave.