  The physical conditions in molecular clouds control the nature and rate of
star formation, with consequences for planet formation and galaxy evolution.
The focus of this review is on the conditions that characterize regions of star
formation in our Galaxy. A review of the tools and tracers for probing physical
conditions includes summaries of generally applicable results. Further
discussion distinguishes between the formation of low-mass stars in relative
isolation and formation in a clustered environment. Evolutionary scenarios and
theoretical predictions are more developed for isolated star formation, and
observational tests are beginning to interact strongly with the theory.
Observers have identified dense cores collapsing to form individual stars or
binaries, and analysis of some of these support theoretical models of collapse.
Stars of both low and high mass form in clustered environments, but massive
stars form almost exclusively in clusters. The theoretical understanding of
such regions is considerably less developed, but observations are providing the
ground rules within which theory must operate. The most rich and massive star
clusters form in massive, dense, turbulent cores, which provide models for star
formation in other galaxies.
