The giant molecular clouds, responsible for virtually all current star formation, are remarkably inefficient in forming stars and are much longer-lived than generally appreciated (~ 108 yrs). The bursts of star formation associated with massive star clusters and starbursts are probably dynamical driven by external motions — spiral arm and bar streaming motions and in the most extreme cases, galactic interactions/merging. We review the properties of molecular gas (GMCs, Molecular Cloud Associations and nuclear gas disks) involved in massive star and cluster formation in the spiral arms of galaxies (e.g. M51), interacting galaxies (e.g. the Antennae), and in ultra-luminous nuclear starbursts (e.g. Arp 220). Both the OB star forming regions (e.g. M51) and the nuclear starbursts such as Arp 220 have approximately the same rate of star formation per unit mass of ISM. We suggest that a maximum efficiency for forming young stars exists — an Eddington-like limit imposed by the radiation pressure of newly formed stars acting on the interstellar dust. This limit corresponds to approximately 500 LSolar/ MSolar for optically thick regions in which the radiation has been degraded to the NIR.
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