molecular cloud) and the stellar initial mass function ($d{\cal N}/dm\propto
m^{-2.35}$, putting most of the mass in young stars into $\la1\,M_\odot$ objects). Similarly, young super star clusters, which tend to have sizes and masses that are generally comparable to the old globulars in the Milky Way, can account for as much as $\sim$20% of the UV light in starburst galaxies (Meurer et al. 1995).

This is, however, [*not*]{} to say that all, or even most, stars are born into bona fide clusters that exist as coherent dynamical units for any length of time; on the contrary, it would appear that very few multiple-star systems actually form as gravitationally bound clusters. (A quantification of “very few” is one of the goals of this paper.) At some point during the collapse and fragmentation of a cluster-sized cloud of gas, the massive stars which it has formed will expel any remaining gas by the combined action of their stellar winds, photoionization, and supernova explosions. If the [*cumulative*]{} star formation efficiency of the cloud, ${\rm SFE}=M_{\rm stars}/ (M_{\rm stars}+M_{\rm gas})$, is below a critical threshold when the gas is lost, then the blow-out carries away sufficient energy that the stellar group remaining is unbound, 