Star formation in molecular clouds?

Using studies of nearby star formation with Spitzer, I will argue that star formation is restricted to dense cores within molecular clouds. The nature of these dense cores and their connection to star formation will be discussed. Their distribution over masses and over the cloud is similar to that of stars, and their efficiency of forming stars is much higher than that of the whole cloud. Moving to regions forming more massive stars, we find that the mass distribution of the dense clumps is similar to that of OB associations. The infrared luminosity per unit mass of dense gas is high and comparable to that seen in starburst galaxies. The relation between star formation and dense gas appears to be linear. Understanding the Kennicutt-Schmidt law requires an understanding of what controls the conversion of gas into the dense entities where stars actually form.