  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.
